The Air Corps issued a circular in March 1938 describing the performance they required from the next bombers — a payload of with a range of at more than. Several American aircraft companies responded with submissions: Bell Model 9 with two engines, Boeing-Stearman Model X-100 with Pratt & Whitney R-2180 radials, Douglas Model 7B with Pratt & Whitney R-1830 Twin Wasp radials, Martin Model 167F and North American submitted their NA-40 design. The NA-40 had benefited from the North American XB-21 of 1936, which was the company's partly successful design for an earlier medium bomber that had been initially accepted and ordered, but then cancelled. However, the company's experience from the XB-21 contributed to the design and development of the NA-40. The single NA-40 built flew first at the end of January 1939. It went through several modifications to correct problems. These improvements included fitting Wright R-2600 "Twin Cyclone" radial engines, in March 1939, which solved the lack of power. In March 1939, North American delivered the substantially redesigned and improved NA-40 to the United StatesArmy Air Corps for evaluation. It was in competition with other manufacturers' designs produced - the Bell had not been built - Douglas 7B, Stearman XA-21, and the Martin Model 167F but failed to win orders. The 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. Despite the loss of the 7B in an accident injuring a French observer in January, the French had ordered the 7B and a revised version. 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 U.S. Army ordered the DB-7 as the A-20 Havoc. The Air Corps issued a specification for a medium bomber in March 1939 that was capable of carrying a payload of over at NAA used the NA-40B design to develop the NA-62, which competed for the medium bomber contract. No YB-25 was available 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".
