Source consulted: Juptner, Joseph P. U.S. Civil Aircraft (Volume I). Aero Publishers, 1962-c1981.
Source consulted: Larkins, William T. The Ford Story: A Pictorial History of the Ford Tri-Motor 1927-1957. R.R. Longo Co., 1958.
Source consulted: Matt, Paul R. "Douglas Mailplanes." From TMOF Tech Files.
Source consulted: National Air Transport. NAT Bulletin Board, no. 19 (September 4, 1928).