The experimental X-15 built by North American Aviation was an astonishing craft. It was both an aircraft and a spaceplane, setting speed and altitude records at the edge of outer space in the 1960s. Such was its performance that some pilots could also hold claim to being astronauts. Even today, the hypersonic X-15 holds the record for the fastest ever manned rocket-powered aircraft at a top speed of Mach 6.32 (a staggering 7,274km/h). It was powered by either two XLR11 or one XLR99 jet engines made by Reaction Motors Inc. To begin its flights, a NASA B-52 would carry the experimental craft aloft before releasing it at an altitude of approximately 14km. Three X-15 aircraft were manufactured and test-flown 199 times, thus creating invaluable data that would help future aircraft and space programs.