A whole series of X-series experimental aircraft were made for the US Air Force (USAF). The first was the Bell X-1, but another one that is equally famous is the North American X-15. The rocket-powered X-15 reached the very edge of outer space, and today it still holds the world record for the fastest speed attained by a manned rocket-powered aircraft. Its fastest recorded speed was Mach 6.72, equating to 7,274km/h. This hypersonic spaceplane was produced by North American Aviation and Reaction Motors, the latter manufacturing the XLR-99 rocket engine. A total of three X-15s were made, and 199 flight tests were conducted from 1959 to 1968. These flights contributed much to American knowledge about aircraft and spacecraft design. For its flight tests, the 15.45m-long X-15 was carried under the wing of a B-52 operated by NASA, and then released at an altitude of nearly 14km.