In this Episode of the Hangar Deck Podcast, the team discusses our Aircraft Pick of the week.  The B-25 Mitchell.  The B-25 was derived from the 1939 NA-40B company proposal that was not accepted by the Army Air Corps.
The Air Corps issued a circular (Number 38-385) in March 1938 describing the performance they required from the next bombers - a payload of 1,200 lb (540 kg) with a range of 1,200 mi (1,900 km) at more than 200 mph (320 km/h). Those performance specifications led NAA to submit their NA-40 design. The NA-40 had benefited from the North Amercian XB-21 (NA-39) 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 though several modifications to correct problems. These improvements included fitting 1,600 hp Wright R-2600 "Double 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 (as NA-40B) to the United States Airmy Air Corps for evaluation. It was in competition with other manufacturers' designs (Douglas 7B, Stearman X-100 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. However, the French had already opted for a revised Douglas 7B (as the DB-7). 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.
The Air Corps issued a specification for a medium bomber in March 1939: 2,400 lb (1,100 kg) over 1,200 mi (1,900 km) at 300 mph (480 km/h) NAA used the NA-40B design to develop the NA-62 which competed for the medium bomber contract. 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 attack type competition, but NAA developed a more advanced design, the NA-40B, which in turn lead to the NA-62, B-25 Mitchell bomber.
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. This design caused stability problems. A slight anhedral on the outboard wing sections nullified the problem and gave the B-25 its 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.
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 and B-25D series at different plants. 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 know 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. The factory produced B-25G entered production during the NA-96 order followed by the redesigned B-25H gunship.The B-25J reverted to the bomber role but it too could be outfitted as a strafer (Pictured above).
North America