It can be argued so, but the MB-1 ("Martin Bomber #1") really is a bomber, sold as a mailplane. Curtiss did the same with it's Carrier Pigeon, a civilized version of the 0-1 Falcon. Military aircraft did not adapt to the role very well and didn't last long. The successful mailplanes were purpose-built as economical commercial aircraft. A lot of them were better airplanes than the military had. Air power was pretty weak at this time.
The military didn't have a very good experience with flying the mail, either. Great story: http://en.wikipedia.org/wiki/Air_Mail_Scandal
(Another great linkup here (1934) in command was Hap Arnold, Ira Eaker and Frank Armstrong -- key players later of the 8th Air Force.)
This mystery ship is a Pennsy-designed and built design.