X-15 aircraft, illustration. The X-15 was a rocket-powered research aircraft designed to investigate flight at hypersonic speeds. Three X-15s flew a total of 199 missions between 1959 and 1968. The X-15 was carried into the air by a converted B-52 bomber and released at high altitude. The pilot then ignited the X-15s rocket motor. It was the first plane to reach the edge of outer space and set records for the fastest manned aircraft (7,274 kilometres per hour, or Mach 6.72).