The North American B-25 Mitchell was an American twin-engined medium bomber manufactured by North American Aviation. It was used by many Allied air forces, in every theater of World War II, as well as many other air forces after the war ended, and saw service across four decades.
The B-25 was named in honor of General Billy Mitchell, a pioneer of U.S. military aviation. By the end of its production, nearly 10,000 B-25s in numerous models had been built. These included a few limited variations, such as the United States Navy’s and Marine Corps’ PBJ-1 patrol bomber and the United States Army Air Forces’ F-10 photo reconnaissance aircraft.
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.
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 11 April 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.
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.
The B-25B variant had it’s tail and gun position removed and replaced by manned dorsal turret on rear fuselage and retractable, remotely operated ventral turret, each with a pair of .50 in (12.7 mm) machine guns. 120 were built, with the version being used in the Doolittle Raid. 23 being supplied to the Royal Air Force as the Mitchell Mk I.