Tham khảo Northrop_Alpha

Ghi chú
  1. Eden and Moeng 2002, pp. 74–77.
Tài liệu
  • Eden, Paul and Soph Moeng. The Complete Encyclopedia of World Aircraft. London: Amber Books Ltd., 2002. ISBN 0-7607-3432-1.
  • Fahey, James C. U.S. Army Aircraft 1908-1946 (Heavier-Than-Air) . New York: Ships and Aircraft, 1946.
  • Smith, M.J. Jr. Passenger Airliners of the United States, 1926-1991. Missoula, Montana: Pictorial Histories Publishing Company, 1986. ISBN 0-933126-72-7.
  • Swanborough, F. Gordon and Peter M. Bowers. United States Military Aircraft Since 1909. New York: Putnam, 1964. ISBN 0-85177-816-X.