of the Douglas aircraft, to be designated B-18A. Despite this, the US Army Air Corps found the performance of the XB-21 to have been favorable enough to order five pre-production aircraft, to be designated YB-21. However, soon after this contract was awarded, it was cancelled, and none of the YB-21s were ever built, leaving the XB-21 as the sole example of the type ever constructed. Operated by North American Aviation, the XB-21 served as a research aircraft until its retirement. Although the XB-21 failed to win a production contract, it was the first of a long line of North American Aviation medium bomber aircraft, and provided experience and knowledge that assisted in the development of the North American NA-40, which, developed into the B-25 Mitchell, would become one of the Army's standard medium bombers of World War II. Specifications (XB-21) See also References Notes Bibliography Donald, David, ed. The Complete Encyclopedia of World Aircraft. London: Orbis, 1997. . Jones, Lloyd S. U.S. Bombers, B1-B70. Fallbrook, California: Aero Publishers, 1962. . Reuter, Claus. Development of Aircraft Turrets in the AAF, 1917–1944. New York: S.R. Research & Publishing, 2000. . Rusinek, Ed. "A Tale of Two Dragons." North American Aviation Retirees Bulletin, Winter 2005. Yenne, Bill. The American Aircraft Factory in World War II. St. Paul, Minnesota: Zenith Press, 2006. . Yenne, Bill. The Story of the Boeing Company. St. Paul, Minnesota: Zenith Press, 2005. . Further reading External links XB-21 at Aero-Web.org 1930s United States bomber aircraft B-21 Aircraft first flown in 1936 Mid-wing aircraft Twin piston-engined tractor aircraft