Technical direction of the X-15 program had been assigned to the National Advisory Committy for Aeronautics (NACA, the predecessor of NASA). Design and construction of the research aircraft had been assigned to the Air Force. The Air Force sent invitation-to-bid letters to 12 Aerospace companies on December 30, 1954. Nine of these companies participated in a bidders' conference oon January 18, 1955.  One day before that conference the Air Force notified NACA that this would be Air Force Project 1226, and that the aircraft designation would be X-15.

Several of the remaining contractors dropped out in the following months, leaving Bell, North Douglas, North American, and Republic as those who submitted proposals for the May 9th deadline to the Air Force at Wright Field (Wright Patterson Air Force Base). On May 11th the Air Force sent technical data for the proposals to evaluation groups at the Wright Air Development Center (WADC), Air Research and Development  Command (ARDC), NACA, and Navy, requesting responses by June 22nd.

Part of the package for evaluation of the proposals was a set of 9 8x10 photographs of design models for all of the proposals. The photo above compares all four proposed designs and a similarly scaled model of the Bell X-1A design, which also had been manufactured as the X-1B and the X-1D.

Here are other photographs in this set, with two for each of the competing design submissions. Click on any photo for a high resolution image.

Bell Aircraft
Douglas Aircraft
North American Aviation
Republic Aircraft

Good references in print for history of design selection for the X-15 (Air Force Project 1226):

Hypersonic!    The Story of the North American X-15
Dennis R. Jenkins
2003, Specialtypress

X-15   The NASA Mission Reports
Robert Godwin, editor
2000,  Apogee Books

Hypersonics Before the Shuttle
Dennis R. Jenkins
2003, NASA Publication SP-2000-4518
(Monographs in Aerospace History, Number 18)

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Documents source:  Original photographic prints in this web author's collection