North American X-15 on display in the “Boeing Milestones of Flight Hall” at the Museum in Washington, DC. The X-15 was the first winged aircraft to attain hypersonic velocities of Mach 4, 5 and 6 (four to six times the speed of sound) and to operate at altitudes well above 30,500 meters (100,000 feet).
Credit: Eric Long, Smithsonian
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