 The first, the Bell X-1, became well known in 1947 after it became the first aircraft to break the sound barrier in level flight.Later X-planes supported important research in a multitude of aerodynamic and technical fields, but only the North American X-15 rocket plane of the early 1960s achieved comparable fame to that of the X-1.X-planes 8, 9, 11, 12, and 17 were actually missiles used to test new types of engines, and some other vehicles were un-manned (some were remotely flown, some were partially or fully autonomous).