The Bell X-1, originally designated XS-1, was a joint NACA-U.S. Army Air Forces-U.S. Air Force supersonic research project built by the Bell Aircraft Company. Conceived in 1944 and designed and built during 1945, it reached nearly 1,000 m.p.h. (1,600 km/h) in 1948. A derivative of this same design, the Bell X-1A, having greater fuel capacity and hence longer rocket burning time, exceeded 1,600 m.p.h. (2,575 km/h) in 1954. The X-1 was the first airplane to exceed the speed of sound in level flight and was the first of the so-called X-planes, an American series of experimental rocket planes designated for testing of new technologies and often kept secret.