B25 prototype...

 On this date, Aug. 19, 1940: At Mines Field (now known as Los Angeles International Airport), the first North American Aviation B-25 twin-engine medium bomber, serial number 40-2165, took off on its first flight with test pilot Vance Breese at the controls and engineer Roy Ferren in the co-pilot’s position. The airplane, North American model NA-62, serial number 62-2834, was developed from two earlier designs which had been evaluated by the U.S. Air Corps but rejected, and it was ordered into production without a prototype being built.

The first few B-25s built — sources vary, but 8-10 airplanes — were built with a constant dihedral wing. Testing at Wright Field showed that the airplane had a slight tendency to “Dutch roll” so all B-25s after those were built with a “cranked” wing, with the outer wing panels having very slight dihedral and giving it the bomber’s characteristic “gull wing” appearance.

The two vertical stabilizers were also increased in size. The B-25 was named Mitchell in honor of early air power advocate Brig. Gen. Billy Mitchell. A total of 9,984 B-25s, F-10 reconnaissance variants and U.S. Navy and Marine Corps PBJ-1 patrol bombers were built by North American Aviation at Inglewood, Calif., and Kansas City, Kansas. The last one, a TB-25J, remained in service with the U.S. Air Force until 1960.