The first airplane that reached Mach 1 was the North American F-86 "Sabre" (during a steep dive). The first airplane that did it in a leveled flight was the Bell X-1.
The North American F-86 Sabre (sometimes called the Sabrejet) was an American transonic jet fighter aircraft. The Sabre is best known for its Korean War role where it was pitted against the Soviet MiG-15 and obtained UN air superiority. Although developed in the late 1940s and outdated by the end of the 1950s, the Sabre proved adaptable and continued as a front line fighter in air forces until the last active front line examples were retired by the Bolivian Air Force in 1994. Its success led to an extended production run of over 7,800 aircraft between 1946 and 1956, in the United States, Japan and Italy. The upgraded Canadair Sabre added another 1,815 airframes, while the CAC CA-27 Avon Sabre saw a production run of 112. It was by far the most-produced Western jet fighter, with total production of all variants at 9,860 units.