P-61 crews trained in a variety of ways. Several existing night fighter squadrons operating in the Mediterranean and Pacific theatres were to transition directly into the P-61 from Bristol Beaufighters and Douglas P-70s, though most P-61 crews were to be made up of new recruits operating in newly commissioned squadrons. After receiving flight, gunnery or radar training in bases around the U.S., the crews were finally assembled and received their P-61 operational training in Florida for transfer to the European Theatre, or California for operations in the Pacific Theatre.

European Theater
The 422d Night Fighter Squadron was the first to complete their training in Florida and, in February 1944, the squadron was shipped to England aboard the RMS Mauretania. The 425th NFS soon followed aboard the RMS Queen Elizabeth.

The situation deteriorated in May 1944 when the squadrons learned that several USAAF generals believed the P-61 was too slow to effectively engage in combat with German fighters and medium bombers. General Spaatz requested de Havilland Mosquito night fighters to equip two U.S. night fighter squadrons based in the UK. The request was denied due to insufficient supplies of Mosquitoes which were in demand for a number of roles. Several pilots in the 422nd NFS threatened to turn in their wings if they were not permitted to fly the Black Widow. At the end of May, the USAAF insisted on a competition between the Mosquito and the P-61 for operation in the European Theater. RAF crews flew the Mosquito Mk XVII while crews from the 422nd NFS flew the P-61. In the end the USAAF determined that the P-61 had a slightly better rate of climb and could turn more tightly than the Mosquito. Colonel Winston Kratz, director of night fighter training in the USAAF, had organized a similar competition earlie r. He said of the results "I'm absolutely sure to this day that the British were lying like troopers. I honestly believe the P-61 was not as fast as the Mosquito, which the British needed because by that time it was the one airplane that could get into Berlin and back without getting shot down. I doubt very seriously that the others knew better. But come what may, the '61 was a good night fighter. In the combat game you've got to be pretty realistic about these things. The P-61 was not a superior night fighter. It was not a poor night fighter. It was a good night fighter. It did not have enough speed".

In England, the 422d NFS finally received their first P-61s in late June, and began flying operational missions over England in mid-July. These aircraft arrived without the dorsal turrets so the squadron's gunners were reassigned to another NFS that was to continue flying the P-70. The first P-61 engagement in the European Theater occurred on July 15 when a P-61 piloted by Lt. Herman Ernst was directed to intercept a V-1 "Buzz Bomb." Diving from above and behind to match the V-1's 350 mph speed, the P-61's plastic rear cone imploded under the pressure and the attack was aborted. The tail cones failed on several early P-61A models before this problem was corrected. On 16 July, Lt. Ernst was again directed to attack a V-1 and, this time, was successful, giving the 422nd NFS and the European Theater its first P-61 kill.

In early August 1944, the 422d NFS transferred to Maupertus, France, and began meeting piloted German aircraft for the first time. A Bf 110 was shot down, and shortly afterwards, the squadron's commanding officer Lieutenant Colonel O. B. Johnson, his P-61 already damaged by anti aircraft land fire, shot down a Fw 190. The 425th NFS scored its first kill shortly afterwards.

In October 1944, a P-61 of the 422nd NFS, now operating out of an abandoned Luftwaffe airfield in Florennes, Belgium, encountered a Messerschmitt Me 163 attempting to land. The P-61 tried to intercept it but the rocket-powered aircraft was gliding too fast. A week later, another P-61 spotted a Messerschmitt Me 262, but was also unable to intercept the jet. On yet another occasion, a 422nd P-61 spotted a Messerschmitt Me 410 Hornisse flying at tree top level but, as they dove on it, the "Hornet" sped away and the P-61 was unable to catch it. Contrary to popular stories, no P-61 ever engaged in combat with a German jet or any of the late war advanced Luftwaffe aircraft. The most commonly encountered and destroyed Luftwaffe aircraft types were Junkers Ju 188s, Junkers Ju 52s, Bf 110s, Fw 190s, Dornier Do 217s and Heinkel He 111s, while P-61 losses were limited to numerous landing accidents, bad weather, friendly and anti aircraft land fire. Apart from exploding V-1s and an attack on a Bf 110 Night Fighter that turned against them, there were no reports of a P-61 being damaged by a German aircraft; and apart from one accidentally shot down by an RAF Mosquito, none were confirmed to be destroyed in aerial combat, though one researcher suggests 42-39515 may have been shot down by an Fw 190 of NSG 9.".

The absence of turrets and gunners in most European Theater P-61s presented several unique challenges. The 422nd NFS kept its radar operator in the rear compartment, meaning the pilot had no visual contact with the R/O. As a result, several courageous pilots continued flying their critically damaged P-61s under the mistaken belief that their R/O was injured and unconscious, when in fact the R/O had already bailed out. The 425th NFS had a more novel solution: they moved the R/O to the former gunner's position behind the pilot. This gave the pilot an extra set of eyes up front, and moved the aircraft's center of gravity about 15 in forward, changing the flight characteristics from slightly nose up to slightly nose down which also improved the P-61's overall performance.

By December 1944, P-61s of the 422nd and 425th NFS were helping to repel the German offensive known as the Battle of the Bulge, with two flying cover over the town of Bastogne. Pilots of the 422nd and 425th NFS switched their tactics from night fighting to daylight ground attack, strafing German supply lines and railroads. The P-61's four 20 mm cannons proved highly effective in destroying large numbers of German locomotives and trucks.

By early 1945, German aircraft were rarely seen and most P-61 night kills were Ju 52s attempting to evacuate German officers under the cover of darkness.

The 422nd NFS produced three ace pilots and two radar operators, while the 425th NFS officially claimed none. Lt. Cletus "Tommy" Ormsby of the 425th NFS was officially credited with three victories. Ormsby was killed by friendly fire moments after attacking two Junkers Ju 87s on the night of 24 March 1945. His radar operator escaped with serious injuries, and was saved only by the quick actions of German surgeons. He later reported that they had successfully engaged and shot down both Ju 87s before being shot down themselves. This claim was corroborated by other 425th aircrew who were operating in the area at the time. To this day, many members of the 425th question why Lt. Ormsby was never credited with his final two kills, and "ace" status.

China-Burma-India (CBI) Theater
P-61s of the China-Burma-India (CBI) Theater were responsible for patrolling a larger area than any night-fighter squadrons of the war. The P-61 arrived too late in the CBI Theater to have any significant impact, as most Japanese aircraft had already been transferred out of the CBI Theater by that time in order to participate in the defense of the Japanese Homeland.

Pacific Theater
The 6th NFS based on Guadalcanal received their first P-61s in early June 1944. The aircraft were quickly assembled and underwent flight testing as the pilots changed from the squadron's aging P-70s. The first operational P-61 mission occurred on 25 June, and the type scored its first kill on 30 June 1944 when a Japanese Mitsubishi G4M "Betty" bomber was shot down.

In the summer of 1944, P-61s in the Pacific Theater saw sporadic action against Japanese aircraft. Most missions ended with no enemy aircraft sighted but when the enemy was detected they were often in groups, with the attack resulting in several kills for that pilot and radar operator, who would jointly receive credit for the kill.

In the Pacific Theater in 1945, P-61 squadrons struggled to find targets. One squadron succeeded in destroying a large number of Kawasaki Ki-48 "Lily" Japanese Army Air Force twin-engined bombers, another shot down several Mitsubishi G4M "Bettys," while another pilot destroyed two Japanese Navy Nakajima J1N1 "Irving" twin-engined fighters in one engagement but most missions were uneventful. Several Pacific Theater squadrons finished the war with no confirmed kills. The 550th could only claim a crippled B-29 Superfortress, shot down after the crew had bailed out having left the aircraft on autopilot.

On 30 January 1945, a lone P-61 performed a vital mission that was instrumental in the successful raid carried out by the U.S. Rangers to free over 500 Allied POWs held by the Japanese at the Cabanatuan prison camp in the Philippines. As the Rangers crept up on the camp, a P-61 swooped low and performed aerobatic maneuvers for several minutes. The distraction of the guards allowed the Rangers to position themselves, undetected within striking range of the camp. The story of the rescue and the role of the P-61 is told in the book Ghost Soldiers (by HamPTOn Sides) and in The Great Raid, a movie based upon the book, though the absence of a flying P-61 forced the filmmakers to feature a Lockheed Hudson in the film in its place.

It was in this theater that poet and novelist James Dickey flew 38 missions as a P-61 radar operator with the 418th Night Fighter Squadron, an experience that profoundly influenced his work, and for which he was awarded five Bronze Stars. The 418th NFS also produced the only AAF night fighter aces in the Pacific, a pilot-radar operator team.

Historian Warren Thompson wrote that "it is widely believed" that the last enemy aircraft destroyed in combat before the Japanese surrender was downed by a P-61B-2 named "Lady in the Dark" (s/n 42-39408) of the 548th NFS. The aircraft piloted by Lt. Robert W. Clyde and R/O Lt. Bruce K. LeFord on 14 August/15 August 1945 claimed a Nakajima Ki-44 "Tojo." The destruction of the "Tojo" came without a shot being fired; after the pilot of the "Tojo" sighted the attacking P-61, he descended to wave-top level and began a series of evasive maneuvers. These ended with his aircraft striking the water and exploding. Lts. Clyde and LeFord were never officially credited with this possible final kill of the war.

Summary
Though the P-61 proved itself very capable against the majority of German aircraft it encountered, it was clearly outclassed by the new aircraft arriving in the last months of World War II. It also lacked external fuel tanks until the last months of the war, an addition that would have extended its range and saved many doomed crews looking for a landing site in darkness and bad weather. External bomb loads would also have made the type more suitable for the ground attack role it soon took on in Europe. These problems were all addressed eventually, but too late to have the impact they might have had earlier in the war. The P-61 proved very capable against all Japanese aircraft it encountered, but saw too few of them to make a significant difference in the Pacific war effort.

Postwar military service
The useful life of the Black Widow was extended for a few years into the immediate postwar period due to the USAAF's problems in developing a useful jet-powered night/all-weather fighter.

In Europe, the United States Air Forces in Europe was organized on 7 August 1945. Its night fighter force was organized with the 415th NFS at AAF Station Nordholz on 2 October; the 417th NFS at AAF Kassel-Rothwesten on 20 August, and the 416th NFS at AAF Station Hörsching, Austria. The 414th, 422d and 425th became non-operational and their personnel were returned to the United States. The 414th's P-61s were transferred to the 416th which was equipped with British de Havilland Mosquitos. High-hour aircraft were scrapped and P-61s in excess of operational needs were mothballed at the Erding Air Depot, Germany. All of these units were inactivated by the end of 1946, personnel and most aircraft being assigned to the 52d Fighter Group. Excess and mothballed Black Widows at Erding were sent to reclamation at Oberpfaffenhofen Air Depot near Munich.

In the Pacific, the 426th, 427th 548th and 550th NFS were inactivated by the end of 1945. As part of the Occupation force in Japan, the 418th and 547th NFS were transferred from Okinawa and Ie Shima to Atsugi Airfield, Japan, and the 421st NFS was reassigned from Ie Shima to Itazuke Airfield, Japan. The 6th, 418th and 421st were all inactivated, their personnel and aircraft being consolidated under the 347th Fighter Group in February 1947. They became the 339th, 4th and 68th Fighter Squadrons respectively. The 419th in the Philippines and the 449th on Guam were both inactivated. Many P-61s in the Pacific that were deemed "war weary" met their fate at reclamation facilities established on Luzon.

P-61s returned to the United States which were considered still operational were organized and allocated to the three new Major Commands established by the 21 March 1946 USAAF reorganization. All of these CONUS-based commands were allocated squadrons which were non-operational that had to be manned and equipped.

To Strategic Air Command the 57th and 58th Reconnaissance Squadrons (Weather) were assigned P-61s. The 57th and 58th NFS had been initially part of Third Air Force, Continental Air Forces and were equipped with early-model P-61Bs that had been used for training pilots in California before being reassigned to Rapid City Army Air Base, South Dakota. Under Third Air Force they were engaged in Weather Reconnaissance training immediately after the war, but the rapid demobilization of the AAF led to the 57th being inactivated by the end of the year, and 58th followed suit in May 1946.

Tactical Air Command was assigned the 415th NFS, and Air Defense Command was assigned the 414th and 425th NFS. The 414th was almost immediately transferred to TAC. Both the 414th and 415th were equipped and manned at Shaw Field, South Carolina and by early 1947 were operationally ready. The 414th was deployed to Caribbean Air Command for defense of the Panama Canal, and the 415th was deployed to Alaskan Air Command for long-range air defense against Soviet aircraft stationed across the Bering Sea in Siberia. Both of these squadrons were soon transferred to the overseas commands by TAC, and were redesignated as Fighter Squadrons.

Air Defense Command organized its Black Widow units with the 425th NFS being reassigned to McChord Field, Washington and the new 318th Fighter Squadron at Mitchel Field, New York in May 1947. A month later, the 52d Fighter Group (with the 2d and 5th Fighter Squadrons) were returned from Germany. With the 52d operational, the 325th Fighter Group at McChord was reassigned to Hamilton Field, near San Francisco with the 317th and 318th squadrons. All of these squadrons were equipped with P-61Bs drawn from storage depots in the southwest. With the change in the USAF's aircraft designation system in June 1948, all P-61s became F-61s and all F-15As became RF-61Cs. Buzz Letters "FH" were assigned.

Ejection seat experiments
A Black Widow participated in early American ejection seat experiments performed shortly after the war. The Germans had pioneered the development of ejection seats early in the war, the first-ever emergency use of an ejection seat having been made on 14 January 1942 by Helmut Schenk, a Luftwaffe test pilot, when he escaped from a disabled Heinkel He 280 V1. American interest in ejection seats during the war was largely a side-issue of the developmental work done on pusher aircraft such as the Vultee XP-54, the goal being to give the pilot at least some slim chance of clearing the tail assembly and the propeller of the aircraft in the case of an emergency escape, but little progress had been made since World War II era pusher aircraft development had never really gotten past the drawing board or the initial prototype stage. However, the development of high-speed jet-powered aircraft made the development of practical ejection seats mandatory.

Initially, an ejection seat was "borrowed" from a captured German Heinkel He 162 and was installed in a Lockheed P-80 Shooting Star in August 1945. However, it was decided that the single-seat P-80 would not be suitable for these tests, and it was decided to switch to a three-seat Black Widow. A P-61B-5-NO (serial number 42-39489) was modified for the tests, the ejection seat being fitted in the forward gunner's compartment. The aircraft was redesignated XP-61B for these tests (there having been no XP-61B prototype for the initial P-61B series). A dummy was used in the initial ejection tests, but on 17 April 1946, a volunteer, Sgt. Lawrence Lambert was successfully ejected from the P-61B at a speed of 302 mph (486 km/h) at 7,800 ft. With the concept having been proven feasible, newer jet-powered aircraft were brought into the program, and the XP-61B was reconverted to standard P-61B configuration.

Thunderstorm project
The P-61 was heavily involved in the Thunderstorm Project (1946–1949) that was a landmark program dedicated to gathering data on thunderstorm activity. The project was a cooperative undertaking on the part of four U.S. government agencies: the U.S. Weather Bureau and the NACA (National Advisory Committee for Aeronautics, later to become NASA), assisted by the U.S. Army Air Forces and Navy. Scientists from several universities also participated in the initiation, design, and conduct of the project. The project's goal was to learn more about thunderstorms and to use this knowledge to better protect civil and military airplanes that operated in their vicinity. The P-61's radar and particular flight characteristics enabled it to find and penetrate the most turbulent regions of a storm, and return crew and instruments intact for detailed study.

The Florida phase of the project in 1946 continued into a second phase carried out in Ohio during the summer of 1947. Results derived from this pioneering field study formed the basis of the scientific understanding of thunderstorms, and much of what was learned has been changed little by subsequent observations and theories. Data was collected for the first time from systematic radar and aircraft penetration of thunderstorms, forming the basis of many published studies that are still frequently referenced by mesoscale and thunderstorm researchers.

Naval tests
P-61B-1NO serial number 42-39458 was operated by the Navy at the Patuxent River test facility in Maryland in a number of tests. P-61A-10NO serial number 42-39395 was subjected by the Navy to a series of test catapult launches to qualify the aircraft for shipboard launches, but the Black Widow was never flown from an aircraft carrier. These aircraft did not receive the naval designation F2T-1 but continued on as P-61.

Shortly after the war, the Navy borrowed two P-61Cs (43-8336 and 43-8347) from the USAAF and used them for air-launches of the experimental Martin PTV-N-2U Gorgon IV ramjet-powered missile, the first launch taking place on 14 November 1947. While carrying a Gorgon under each wing, the P-61C would go into a slight dive during launch to reach the speed necessary for the ramjet to start. These two naval Black Widows were returned to the USAF in 1948, and transferred to the boneyard shortly afterwards.

Retirement
In 1945 the USAAF programmed a jet night Interceptor to replace the F-61. To meet the jet-powered night fighter requirement, Curtiss-Wright proposed an aircraft of a similar configuration, but adapted specifically for the interception role. The company designation of Model 29A was assigned to the project. The Army ordered two prototypes under the designation Curtiss-Wright XF-87 Blackhawk and the name "Blackhawk" was assigned. However, the USAAF also thought highly of the Northrop proposal, which was given the designation N-24 by the company. Two prototypes were ordered under the designation XP-89 in December 1946.

Development delays in both the XF-89 and XF-87 projects meant that the F-61 Black Widows still in service in 1947 were rapidly reaching the end of their operational lifetimes. They had been built for wartime duty, and at most, had been expected to be in service only for a year or two until being replaced by jets. No plans for long-term use had been made, and a parts shortage meant that those aircraft still in service were being supported by cannibalization of stored aircraft at Davis-Monthan and other storage depots. In early 1948, the USAF ordered that a flyoff take place between the Northrop XF-89, the Curtiss XF-87, and the Navy's Douglas XF3D-1 Skyknight. The evaluation team judged the XF-89 as being the superior fighter and having the best development potential, and the F-87A order was cancelled in its entirety on 10 October. The F-89s finally reached USAF service in 1951.

An interim replacement was found with the F-82 Twin Mustang, whose engineless airframes were sitting in storage at North American Aviation in California and could be put into service quickly. Retirement of the F-61 began in 1948 by F-82s equipped as night fighters, and by the end of the year all of the ADC Black Widows in the United States, Alaska and in Panama were off the inventory rolls. Most of Far East Air Force's F-61s were retired in 1949; the last operational Black Widow of the 68th Fighter Squadron, 347th Fighter Group left Japan in May 1950, missing the Korean War by only a month.

In 1948, the National Advisory Committee for Aeronautics (NACA) obtained an F-61C from Air Research and Development Command for a series of drop tests of swept-wing aerodynamic drones at Moffett Field, California. Much engineering data was obtained from these tests. RF-61C 45-59300 thus became the last operational USAF F-61 to be retired at the end of the NACA testing in 1953. A second F-51C (43-8330) which was still flyable was obtained from the Smithsonian Institution by NACA in October 1950 for these tests, and remained in use by NACA until 9 August 1954, being the last F-61 in government use. This aircraft is now on public display at the NASM's Steven F. Udvar-Hazy Center. F-61B-15NO serial number 42-39754 was used by NACA's Lewis Flight Propulsion Laboratory in Cleveland, Ohio for tests of airfoil-type ramjets. F-61C-1NO 43-8357 was used at Ames as a source for spare parts for other F/RF-61 aircraft.

Civilian use
Surviving aircraft were offered to civilian governmental agencies, or declared surplus and offered for sale on the commercial market. Five were eventually issued civil registrations

P-61B-1NO serial number 42-39419 had been bailed to Northrop during most of its military career, who then bought the aircraft from the government at the end of the war. Having the civilian registration number NX30020 assigned to it, it was used as an executive transport, as a flight-test chase plane, and for tests with advanced navigational equipment. Later it was purchased by the Jack Ammann Photogrammetric Engineers, a photo-mapping company based in Texas; then in 1963, it was sold to an aerial tanker company and used for fighting forest fires. However, it crashed while fighting a fire on 23 August 1963, killing its pilot.

Last flight
The last flying example of the P-61 line was a rare F-15A Reporter (RF-61C) (s/n 45-59300), the first production model Reporter to be built. The aircraft was completed on 15 May 1946, and served with the USAAF and later the U.S. Air Force until 6 February 1948, when it was reassigned to the Ames Aeronautical Laboratory at Moffett Field in California, where it was reconfigured to serve as a launch vehicle for air dropped scale models of experimental aircraft. It served in this capacity until 1953, when it was replaced by a mammoth wind tunnel used for the same testing. In April 1955, the F-15 was declared surplus along with a "spare parts" F-61C (s/n 43-8357). The F-15 was sold, along with the parts P-61, to Steward-Davis Incorporated of Gardena, California, and given the civilian registration N5093V. Unable to sell it, the P-61C was scrapped in 1957. Steward-Davis made several modifications to the Reporter to make it suitable for aerial survey work, including switching to a canopy tak en from a Lockheed T-33 Shooting Star, and to propellers taken from an older P-61. The aircraft was sold in September 1956 to Compania Mexicana Aerofoto S. A. of Mexico City and assigned the Mexican registration XB-FUJ. In Mexico, the Reporter was used for aerial survey work, the very role for which it was originally designed. It was bought by Aero Enterprises Inc. of Willets, California and returned to the USA in January 1964 carrying the civilian registration number N9768Z. The fuselage tank and turbosupercharger intercoolers were removed; and the aircraft was fitted with a 1,600 gal (6,056 l) chemical tank for fire-fighting. It was purchased by Cal-Nat of Fresno, California at the end of 1964, which operated it as a firefighting aircraft for the next 3½ years. In March 1968, the F-15 was purchased by TBM, Inc., an aerial firefighting company located in Tulare, California (the name of the company standing for the TBM Avenger, the company's primary equipment), who performed additiona l modifications on the aircraft to improve its performance, including experimenting with several types of propellers before deciding on Curtiss Electric type 34 propellers taken from a late model Lockheed Constellation.

On 6 September 1968, Ralph Ponte, one of three civilian pilots to hold a rating for the F-15, was flying a series of routine Phoscheck drops on a fire raging near Hollister, California. In an effort to reduce his return time, Ponte opted to reload at a small airfield nearer the fire. The runway was shorter than the one in Fresno, and despite reducing his load, hot air from the nearby fire reduced the surrounding air pressure and rendered the aircraft overweight. Even at full power the Reporter had not rotated after clearing the 3,500 ft (1,067 m) marker, and Ponte quickly decided to abort his takeoff. Every effort was made to control the hurtling craft, but the Reporter careered off the runway and through a vegetable patch, before striking an embankment which tore off the landing gear. The aircraft then slid sideways, broke up and caught fire. Ponte scrambled through the shattered canopy unhurt, while a firefighting Avenger dropped its load of Phoscheck on the aircraft's two engines, possibly saving Ponte's life. The F-15, though intact, was deemed too badly damaged to rebuild, and was soon scrapped, bringing an end to the career of one of Northrop's most successful designs.