In May, 1946, the Nuclear Energy for the Propulsion of Aircraft (NEPA) project was started by the Air Force. Studies under this program were done until May, 1951 when NEPA was replaced by the Aircraft Nuclear Propulsion (ANP) program. The ANP program contained plans for two B-36s to be modified by Convair under the MX-1589 project. One of the B-36s was to be used to study shielding requirements for an airborne reactor while the other was to be the X-6.