North American Aviation's first twin-engined military aircraft,[REF] the NA-21 prototype was constructed at North American's factory in Inglewood, California,[REF] where work on the aircraft began in early 1936 [REF].
Flown by a crew of six to eight men,[REF] the XB-21 featured a remarkably strong defensive armament for the time,[REF] including as many as five .30-calibre