The idea of a nuclear-powered aircraft (with a duration measured in days rather than in hours) dates back to the late 1940s. In 1946, the Air Force gave a contract to the Fairchild Engine and Airplane Company to explore the possibility of a nuclear-powered aircraft. The study was known by the name NEPA (which stood for Nuclear Energy for the Propulsion of Aircraft) and was carried out at Oak Ridge in Tennessee. In 1948, another study was performed for the Atomic Energy Commission by the Massachusetts Institute of Technology. This study concluded that a nuclear-powered aircraft was actually feasible, but that it would take at least 15 years to develop.
By early 1951, the Air Force had concluded that the NEPA project had shown enough promise that work should begin on an actual propulsion unit. General Electric was the prime propulsion contractor. The system would work by having air enter a compressor, where it would be heated by passing through the reactor, and be exhausted through a jet nozzle. Pratt and Whitney was given a contract for the development of an indirect cycle engine which would use an intermediate fluid to transfer the heat to the air rather than by passing the air through the reactor core itself.
In 1954, the Air Force decided to begin work on an actual aircraft, given the project name of WS-125A. Pratt & Whitney and General Electric were to be the primary engine contractors, with Lockheed and Convair handling the airframe work. The WS-125A would be a high-altitude subsonic bomber, but would have a supersonic cruise capability.
As part of the project, it was necessary to test the effects of nuclear reactor radiation on instruments, equipment, and airframe and to study shielding methods. To support this effort, B-36H ser no 51-5712 was assigned to the program on May 11, 1953. This particular aircraft had been severely damaged in a tornado which struck Carswell AFB on September 1, 1952. Rather than trying to repair the heavily-damaged nose section, the plane was kept by Convair and reassigned to the ANP program.
A nuclear reactor (which did not actually power the aircraft) was mounted in the aft bomb bay. The reactor was a 1000-kilowatt design weighing 35,000 pounds. The reactor could be removed from the aircraft by a crane while on the ground. A number of large air intake and exhaust holes were installed in the sides and bottom of the rear fuselage to cool the reactor. The crew of 5 (pilot, copilot, flight engineer, plus two nuclear engineers) was housed entirely in a highly-modified compartment in the fuselage nose section. The compartment was composed of lead and rubber, and entirely surrounded the crew. A four-ton lead disc shield was installed in the middle of the aircraft. Only the pilot and co-pilot could see out through the foot-thick, leaded-glass windshield. A closed-circuit television system enabled the crew to watch the reactor. The aircraft was redesignated XB-36H. It bore the name "Crusader" on the fuselage side.
Its first flight was made on September 17, 1955, with test pilot A. S. Witchell, Jr. at the controls. All of the test flights were carried out over sparsely-populated areas, and the reactor was not turned on until the plane was at a safe altitude. Flying alongside the XB-36H on every one of its flights was a C-97 transport carrying a platoon of armed Marines ready to parachute down and surround the test aircraft in case it crashed. The project was classified until late 1955 when the Defense Department revealed the existence of the B-36 testbed.
In the autumn of 1956, the aircraft was redesignated NB-36H. However, at about this time, the Air Force decided to cancel the WS-125A nuclear aircraft program. The NB-36H made its last flight on March 28, 157. Up to that time, a total of 47 flights had been made. The NB-36H was decommissioned at Fort Worth in late 1957. It was scrapped several months later, with the radioactive parts being buried.
A nuclear-powered B-36H, which was temporarily designated X-6, had been ordered in 1951. However, it never actually materialized.