The North American X-15 was a rocket-powered research aircraft flown jointly by the U.S. Air Force and NASA between 1959 and 1968. Air-launched from a modified B-52 bomber and powered by a throttleable rocket engine, the X-15 reached speeds exceeding Mach 6 and altitudes above 350,000 feet, blurring the line between aviation and spaceflight. Data gathered from its flights informed later human spaceflight and Space Shuttle development, cementing its place as one of the most significant experimental aircraft of the twentieth century. Project: 218215