Form the Fact Sheet:
[i]In powered flights, a rocket engine accelerated the X-24B to more than 1,000 mph as it climbed to altitudes around 60,000-70,000 feet. The X-24B then made steep unpowered gliding landings like the future space shuttle. Highlights of the X-24B research program included two precise landings made on a concrete runway at Edwards AFB, Calif. (other flights landed on nearby dry lake beds). This ability to glide to a landing at a specific spot was an important step toward later space shuttle operations. The X-24B was the last joint USAF/NASA rocket-powered air-launched research aircraft.[/i]

http://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195762/martin-x-24b/