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B-25 Mitchell in flight during WW II.

B-25 Mitchell in flight during WW II.

B-25 Mitchell – Development:
In 1936, North American Aviation began work on its first twin-engine military design. Dubbed the NA-21 (later NA-39), this project ultimately evolved into the XB-21. In testing against the Douglas B-18 Bolo, the design performed well but a high price tag led the US Army Air Corps to pass on the aircraft. Utilizing the lessons learned from the project, North American moved forward with a new design for a medium bomber which was dubbed the NA-40. Among the improvements incorporated in the NA-40 was the use of two Wright R-2600 Cyclone engines.
Seeking to take advantage of Britain and France’s need for a medium bomber during the early days of World War II, North American originally intended the NA-40 to be built for export. Both countries elected to move forward with a different aircraft and North American submitted an improved NA-40B design to the USAAC for evaluation as a medium bomber. A prototype of the aircraft first flew on August 19, 1940. As the USAAC had an immediate need for a medium bomber, the type was accepted without additional test versions being built.