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NORTH AMERICAN B-25 MITCHELL
Design & Development
The B-25 was a descendant of the earlier
XB-21 (North American-39) project of the mid-1930s. Experience gained in developing that aircraft was eventually used by North American in designing the B-25 (called the NA-40 by the company). One NA-40 was built, with several modifications later being done to test a number of potential improvements. These improvements included Wright R-2600 radial engines, which would become standard on the later B-25.
North American XB-21.
[Source: Unknown]
In 1939, the modified and improved NA-40B was submitted to the United States Army Air Corps for evaluation. This aircraft was originally intended to be an attack bomber for export to the United Kingdom and France, both of which had a pressing requirement for such aircraft in the early stages of World War II. However, those countries changed their minds, opting instead for the also-new Douglas DB-7 (later to be used by the U.S. as the
A-20 Havoc). Despite this loss of sales, the NA-40B re-entered the spotlight when the Army Air Corps evaluated it for use as a medium bomber. Unfortunately, the NA-40B was destroyed in a crash on April 11, 1939. Nonetheless, the type was ordered into production, along with the Army's other new medium bomber, the Martin B-26 Marauder.
An improvement of the NA-40B, dubbed the NA-62, was the basis for the first actual B-25. Due to the pressing need for medium bombers by the Army, no experimental or service-test versions were built. Any necessary modifications were made during production runs, or to existing aircraft at field modification centers around the world.
Assembling the North American B-25 Mitchell at Kansas City, KS, October 1942.
[Source: National Archives]
A significant change in the early days of B-25 production was a redesign of the wing. In the first nine aircraft, a constant-dihedral wing was used, in which the wing had a consistent, straight, slight upward angle from the fuselage to the wing tip. This design caused stability problems, and as a result, the dihedral angle was nullified on the outboard wing sections, giving the B-25 its slightly gull wing configuration. Less noticeable changes during this period included an increase in the size of the tail fins and a decrease in their inward cant.
B-25 Mitchell undergoing final assembly in Inglewood, CA.
[Source: National Archives]
The B-25 was a safe and forgiving aircraft to fly. With an engine out, 60° banking turns into the dead engine were possible, and control could be easily maintained down to 145 mph (230 km/h). However, the pilot had to remember to maintain engine-out directional control at low speeds after takeoff with rudder; if this maneuver was attempted with ailerons, the aircraft would snap out of control. The tricycle landing gear made for excellent visibility while taxiing. The only significant complaint about the B-25 was the extremely high noise level produced by its engines; as a result, many pilots eventually suffered from various degrees of hearing loss. The high noise level was due to design and space restrictions in the engine cowlings which resulted in the exhaust "stacks" protruding directly from the cowling ring and partly covered by a small triangular fairing. This arrangement directed exhaust and noise directly at the pilot and crew compartments. Crew members and operators on the
airshow circuit frequently comment that "the B-25 is the fastest way to turn aviation fuel directly into noise". Many B-25s now in civilian ownership have been modified with exhaust rings that direct the exhaust through the outboard bottom section of the cowling.
[Source: Unknown]
The Mitchell was an amazingly sturdy aircraft that could withstand tremendous punishment. One well-known B-25C of the 321st Bomb Group was nicknamed "Patches" because its crew chief painted all the aircraft's flak hole patches with high-visibility zinc chromate primer. By the end of the war, this aircraft had completed over 300 missions, was belly-landed six times and sported over 400 patched holes. The airframe was so bent askew that straight-and-level flight required 8° of left aileron trim and 6° of right rudder, causing the aircraft to "crab" sideways across the sky.
[Source: Unknown]
An interesting characteristic of the B-25 was its ability to extend range by using one-quarter wing flap settings. Since the aircraft normally cruised in a slightly nose-high attitude, about 40 gal (150 l) of fuel was below the fuel pickup point and thus unavailable for use. The flaps-down setting gave the aircraft a more level flight attitude, which resulted in this fuel becoming available, thus slightly extending the aircraft's range.
Source:
Wikipedia
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