Source consulted: Davies, R. E. G. (Ronald Edward George). Airlines of the United States since 1914. Putnam, 1972.
Source consulted: Francillon, René J. McDonnell Douglas Aircraft Since 1920. Putnam, 1979.
Source consulted: Matt, Paul R. "Douglas Mailplanes." From TMOF Tech Files.
Appears in: National Air Transport. NAT Bulletin Board, no. 5 (August 19, 1927).