The Bell X-1, originally designated XS-1, was a joint NACA/US Army/US Air Force supersonic research project built by Bell Aircraft. It 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.
The X-1 was in principle a "bullet with wings", its shape closely resembled the Browning .50-caliber machine gun bullet that was known to be stable in supersonic flight. The pattern shape was followed to the point of seating the pilot behind a sloped, framed window inside a confined cockpit in the nose, with no ejection seat.
The rocket propulsion system was a four-chamber engine built by Reaction Motors, Inc., one of the first companies to build liquid-propellant rocket engines in America. It burned ethyl alcohol diluted with water and liquid oxygen. The thrust could be changed in 1,500 lbf (6,700 N) increments by firing one or more of the chambers. The fuel and oxygen tanks for the first two X-1 engines were pressurized with nitrogen and the rest with steam-driven turbopumps. The all-important fuel turbopumps, necessary to raise the chamber pressure and thrust, while lightening the engine, were built by Robert Goddard who was under Navy contract to provide jet-assisted takeoff rockets.
On 14 October 1947, just under a month after the United States Air Force had been created as a separate service, the tests culminated in the first manned supersonic flight, piloted by Air Force Captain Charles "Chuck" Yeager in aircraft #46-062, which he had christened Glamorous Glennis after his wife. The rocket-powered aircraft was launched from the bomb bay of a specially modified B-29 and glided to a landing on a runway. XS-1 flight number 50 is the first one where the X-1 recorded supersonic flight, at Mach 1.06 (361 m/s, 1,299 km/h, 807.2 mph) peak speed.
Source: Wikipedia, "Bell X-1", available under the CC-BY-SA License.