In January 1943, a contract for two prototypes under the XP-79 designation was issued by the United States Army Air Forces (USAAF).

It was planned to use a 2,000 lbf (8,900 N) thrust XCALR-2000A-1 "rotojet" rocket motor from Aerojet that used mono-ethylaniline fuel and red fuming nitric acid (RFNA) oxidizer.

The pilot controlled the XP-79 through a tiller bar and intakes mounted at the wingtips supplied air for the unusual bellows-boosted split elevons which opened differentially to provide lateral (yaw) control, by increasing drag on one side of the aircraft, in addition to providing roll and pitch.

Completed in late Spring 1943, MX-334 No.1 was tested in NACA Langley's wind tunnel, after which a large wire-braced fin was added for directional stability at high speeds.

The first flight attempts were carried out by the no.2 aircraft towed behind a Cadillac car for low level take-off and landing tests, with no success.

Testing with the rocket motor commenced on 22 June 1944, with the first aerotow launch for a powered flight on 5 July 1944, making it the first US-built rocket-powered aircraft to fly.