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The Bell X-1, originally designated XS-1, was a joint NACA-U.S. Army/US Air Force supersonic research project built by Bell Aircraft. Conceived in 1944 and designed and built over 1945, it eventually reached nearly 1,000 mph (1,609 km/h) in 1948. A derivative of this same design, the Bell X-1A, having greater fuel capacity and thus longer engine-burn time, exceeded 1,600 mph (2,574 km/h) in 1954. The X-1 was the first aircraft to exceed the speed of sound in controlled, level flight, and 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.