The Bell X-1, originally designated as XS-1, was a joint NACA-U.S. (now NASA) Army Air Forces/US Air Force supersonic research project and the first aircraft to exceed the speed of sound in controlled, level flight. It was the first of the so-called X-planes, an American series of experimental aircraft designated for testing of new technologies and usually kept highly secret.

the X-1A (serial 48-1384) was intended to investigate aerodynamic phenomena at speeds above Mach 2 (681 m/s, 2,451 km/h) and altitudes greater than 90,000 feet (27 km), specifically focusing on dynamic stability and air loads. Longer and heavier than the original X-1 with a bubble canopy for better vision, the X-1A was powered by the same Reaction Motors XLR-11 rocket engine. The aircraft first flew, unpowered, on 14 February 1953 at Edwards AFB, with the first powered flight on 21 February. Both flights were piloted by Bell test pilot Jean Ziegler. The aircraft was transferred to NACA in September 1954. Following modifications, including the installation of an ejection seat, the aircraft was lost on August 8, 1955 while being prepared for launch from the RB-50 mothership.

The XS-1 was the first high-speed aircraft built purely for aviation research purposes and was never intended for production.