The Bell X-1 was the first piloted aircraft to exceed the speed of sound in level flight, in 1947.
On October 14, 1947, U.S. Air Force test pilot Chuck Yeager flew the rocket-powered Bell X-1, aircraft number 46-062, over California's Mojave Desert. The aircraft reached about Mach 1.06, demonstrating that a piloted airplane could safely pass through the difficult transonic region. It was nicknamed Glamorous Glennis after Yeager's wife.
The X-1 was not a conventional jet aircraft. Its rocket engine provided the intense thrust needed for the attempt, and a B-29 bomber carried it to altitude before releasing it. The aircraft's thin, bullet-shaped fuselage was modeled partly on a .50-caliber machine-gun bullet, whose stability at high speed had been studied.
The question is sometimes confused with the first jet aircraft or with the first supersonic passenger aircraft. The German Heinkel He 178 was the first jet-powered aircraft to fly, while Concorde later became a famous supersonic airliner. The Bell X-1's distinction was the first controlled, piloted level flight beyond Mach 1.