The North American B-25 Mitchell is an American twin-engine, medium bomber manufactured by North American Aviation. It was named in honor of Major General William Billy Mitchell, used by many Allied air forces, the B-25 served in every theater of World War II and after the war ended many remained in service, operating across four decades. Produced in numerous variants, nearly 10,000 Mitchells rolled from NAA factories and these included a few limited models, such as the United States Marine Corps PBJ-1 patrol bomber and the United States Army Air Forces F-10 reconnaissance aircraft and AT-24 trainers. The Air Corps issued a circular in March 1938 describing the performance they required from the next bombers — a payload of 1,200 lb with a range of 1,200 mi at more than 200 mph and those performance specifications led NAA to submit their NA-40 design. However, the experience from the XB-21 contributed to the design. The single NA-40 built flew first at the end of January 1939 and it went through several modifications to correct problems. These improvements included fitting 1,600 hp Wright R-2600 Twin Cyclone radial engines, in March 1939, in March 1939, North American delivered the substantially redesigned and improved NA-40 to the United States Army Air Corps for evaluation. It was in competition with other manufacturers designs but failed to win orders, however, the French had already opted for a revised Douglas 7B. Unfortunately, the NA-40B was destroyed in a crash on 11 April 1939 while undergoing testing, although the crash was not considered due to a fault with the aircraft design, the Army ordered the DB-7 as the A-20. There was no YB-25 for prototype service tests, in September 1939, the Air Corps ordered the NA-62 into production as the B-25, along with the other new Air Corps medium bomber, the Martin B-26 Marauder off the drawing board. The NA-40 lost out to the Douglas A-20 in the competition, but NAA developed a more advanced design, the NA-40B. Early into B-25 production, NAA incorporated a significant redesign to the wing dihedral, the first nine aircraft had a constant-dihedral, meaning the wing had a consistent, upward angle from the fuselage to the wingtip. Flattening the outer wing panels by giving them a slight anhedral angle just outboard of the engine nacelles nullified the problem, less noticeable changes during this period included an increase in the size of the tail fins and a decrease in their inward tilt at their tops. NAA continued design and development in 1940 and 1941, both the B-25A and B-25B series entered AAF service. The B-25B was operational in 1942, combat requirements lead to further developments. Before the year was over, NAA was producing the B-25C, also in 1942, the manufacturer began design work on the cannon-armed B-25G series. The NA-100 of 1943 and 1944 was an interim armament development at the Kansas City complex known as the B-25D2, Similar armament upgrades by U. S-based commercial modification centers involved about half of the B-25G series. Further development led to the B-25H, B-25J, and B-25J2, the gunship design concept dates to late 1942 and NAA sent a field technical representative to the SWPA
A B-25 Mitchell in 2007
Nose-on view of the NA-40 prototype, showing the constant dihedral wing design discarded in early development of the successor B-25 design.