The new Model 32 combined designer David R. Davis's wing, a high-efficiency airfoil design created by unorthodox means, with the twin tail design from the Consolidated Model 31 flying boat, together on a new fuselage. This new fuselage was intentionally designed around twin bomb bays, each one being the same size and capacity of the B-17 bomb bays.

In January 1939, the USAAC, under Specification C-212, formally invited Consolidated to submit a design study for a bomber with longer range, higher speed and greater ceiling than the B-17. The specification was written such that the Model 32 would automatically be the winning design. The program was run under the umbrella group, "Project A", an Air Corps requirement for an intercontinental bomber that had been conceived in the mid-1930s. Although the B-24 did not meet Project A goals, it was a step in that direction. Project A led to the development of the Boeing B-29 and Consolidated's own B-32 and B-36.

Design
The B-24 had a shoulder mounted high aspect ratio Davis wing. This wing was highly efficient allowing a relatively high airspeed and long range. Compared to the B-17 it had a 6-foot larger wingspan, but a lower wing area. This gave the B-24 a 35% higher wing loading. The relatively thick wing held the promise of increased tankage while delivering increased lift and speed, but became unpleasant to fly when committed to heavier loadings as experienced at high altitude and in bad weather. The Davis wing was also more susceptible to ice formation than contemporary designs, causing distortions of the aerofoil section and resulting in the loss of lift (unpleasant experiences drawing such comments as 'The Davis wing won't hold enough ice to chill your drink'.) The wing was also more susceptible to damage than the B-17's wing, making the aircraft less able to absorb battle damage. The wing carried four supercharged radial engines mounted in cowlings borrowed from the PBY Catalina (except being oval in cross-section, with oil coolers mounted on each side of the engine), turning 3-bladed variable-pitch propellers.

The tail plane featured two large oval vertical stabilizers mounted at the ends of a rectangular horizontal stabilizer. As early as 1942, it was recognized that the Liberator's handling and stability could be improved by the use of a single vertical fin. The single fin was tested by Ford on the single B-24ST and an experimental XB-24K, and was found to improve handling. All Liberators were produced with twin oval fins, with the exception of eight preproduction B-24N aircraft. The B-24N was intended as a major production variant featuring a single tail. Over 5000 orders for this version were placed in 1945, but were cancelled due to the end of the war. The single fin did appear in production on the PB4Y Privateer derivative.

The B-24's spacious, slab-sided fuselage (which earned the aircraft the nickname "Flying Boxcar") was built around two central bomb bays that could accommodate up to 8,000 pounds (3,600 kg) of ordnance in each compartment (but rarely did, as this decreased range and altitude). The forward and aft bomb bay compartments were further split longitudinally with a centerline ventral catwalk just nine inches (23 cm) wide, which also functioned as the fuselage's structural keel beam. An unusual four-panel set of all-metal, tambour-panel "roller-type" bomb bay doors, which operated very much like the movable enclosure of a rolltop desk, retracted into the fuselage, creating a minimum of aerodynamic drag to keep speed high over the target area, and also allowed the bomb bays to be opened while on the ground; the low ground clearance prevented the use of normal bomb bay doors. The occasional need for crewmen to move around inside from fore to aft within the B-24's fuselage during a mission over the narrow catwalk was a drawback shared with other designs.

The Liberator carried a crew of up to 10. The pilot and co-pilot sat alongside each other in a well glazed cockpit. The navigator and bombardier,who could also double as a nose or wiggly ear gunner[clarification needed], sat in the nose, fronted on the pre-B-24H models with a well-framed "greenhouse" nose with some two dozen glazed panels in total, with two flexible ball-mounts built into it for forward defensive firepower using .30 caliber (7.62 mm) Browning M1919 machine guns. Later versions were fitted with a powered twin- .50 caliber (12.7 mm) M2 Browning machine gun nose turret The radio/radar operator sat behind the pilots, facing sideways and sometimes doubled as a waist gunner. The upper gun turret, when fitted, was located just behind the cockpit, in front of the wing, and was operated by the flight engineer, who sat adjacent to the radio operator behind the pilots. In the tail, up to four crew could be located in the waist, operating waist guns, a retractable lower ball turret and a tail gun turret matching the nose turret. The waist gun hatches were provided with doors, with the ball turret required to be retractable for ground clearance when preparing to land, as well as for greater aerodynamic efficiency. The tail gunner's powered twin-gun turret was located at the end of the tail, behind the tailplane.

The B-24 featured a tricycle undercarriage, the first American bomber to do so, with the main gear extending out of the wing on long, single-oleo strut legs. It used differential braking and differential thrust for ground steering, which made taxiing difficult.

Armament
The defensive armament of the B-24 varied from transport variants, which were usually unarmed, to bombers armed with up to 10 .50 caliber (12.7 mm) M2 Browning machine guns located in turrets and waist gun positions.

Early model Liberators were fitted with a top mounted turret, a tail turret and single machine guns located in the waist and in the glazed nose. The B-24 D initially featured upper, belly and tail turrets, plus swiveling single guns in the waist and on either side of the nose. The belly turret was a periscopically sighted Bendix model. The turret proved unsatisfactory and was soon replaced by a tunnel gun, which was itself omitted. Later D models were fitted with the retractable Sperry Ball turret.

Ball turret of a RAF Coastal Command Liberator.
[Source: Imperial War Museum Collection]

Long range naval patrol versions often carried a light defensive armament. Being on long distance patrols, they generally flew outside the range of enemy fighters. Also, the necessity of range increased the importance of weight and aerodynamic efficiency. Thus naval patrol often omitted top, belly and nose turrets. Some were fitted with belly pack containing fixed, forward facing cannon.

Prototypes and service evaluation
The U.S. Army Air Corps awarded a contract for the prototype XB-24 in March 1939, with the requirement that one example should be ready before the end of the year. Consolidated finished the prototype and had it ready for its first flight two days before the end of 1939. The design was simple in concept but, nevertheless, advanced for its time. Consolidated incorporated innovative features such as a tricycle landing gear and Davis wing.

Compared to the B-17, the proposed Model 32 had a shorter fuselage and 25% less wing area, but had a 6 ft (1.8 m) greater wingspan and a substantially larger carrying capacity, as well as a distinctive twin tail. Whereas the B-17 used 9-cylinder Wright R-1820 Cyclone engines, the Consolidated design used twin-row, 14-cylinder Pratt & Whitney R-1830 "Twin Wasp" radials of 1,000 hp (750 kW). The 70,547 pounds (32,000 kg) maximum takeoff weight was one of the highest of the period.

B-24 Liberator engine nacelle.
[Source: Scott Rose/Warbirds Resource Group]

Early orders, placed before the XB-24 had flown, included 36 for the USAAC, 120 for the French Air Force and 164 for the Royal Air Force (RAF). The name "Liberator" was originally given to it by the RAF, and subsequently adopted by the USAAF as the official name for the Model 24. When France fell in 1940, their aircraft were re-directed to the RAF. One outcome of the British and French purchasing commissions was a backlog of orders amounting to $680m, of which $400 were foreign orders, US official statistics indicating tooling, plant and expansion advanced the previously anticipated volume of US aircraft production by up to a year. A consequence of the British orders went beyond requests for specific modifications: as the RAF accepted some designs while rejecting others, American production was - to some extent - re-directed along specific lines that accorded with British doctrine, the B-24's capacious bomb bay and ability to carry 8,000 lb ordnance a case in point.

After initial testing, the XB-24 was found to be deficient in several areas. One major failure of the prototype was that it failed to meet the top speed requirements specified in the contract. As built, the XB-24 top speed was only 273 mph instead of the specified 311 mph. As a result, the mechanically supercharged Pratt & Whitney R-1830-33s were replaced with the turbo-supercharged R-1830s. Additionally, the tail span was widened by 2 ft (0.61 m) and the pitot-static probes were relocated from the wings to the fuselage. The XB-24 was then re-designated XB-24B—these changes became standard on all B-24s built starting with the B-24C model.

Consolidated XB-24.
[Source: USAF Photo]