In 1942 Jack Northrop proposed to the Army Air Force a super high-performance, flying wing, rocket-powered manned interceptor. Development was authorised in January 1943 under extreme secrecy, the official XP-79 designation being assigned but the work being simply known as 'Project X'. GALCIT at the California Institute of Technology, the only organization in the United States capable of such work, began parallel design of two liquid propellant rocket engine variants for the XP-79 in November 1942. One, the XLR-7, used what would become the later standard gas turbine pump feeding four combustion chambers. The other, using an ancient Greek concept, was dubbed the Aerotojet. The Aerojet company was formed to handle the Project X and JATO government contracts coming GALCIT's way.

The XLR-7 was regeneratively cooled and used 4 x 680 kgf thrust chambers of the type developed by Aerojet for JATO units. The engines had variable thrust, using a gas generator to drive the pumps and a turborocket for main aircraft propulsion. Protracted technical problems and wartime shortages of skilled engineering staff and materials stretched the development schedule. The Aerotojet was abandoned after insoluble propellant leakage problems and the explosion of the first flight version on the test stand in 1944. The XLR-7, on the other hand, was successfully tested in August 1945. But then the P-79 aircraft itself was then cancelled at the end of 1945 when the prototype crashed on its first flight, killing the pilot.