The X-15 was a rocket-powered aircraft developed by NASA and the U.S. Air Force in the 1960s. It was designed to explore the boundaries of flight at high altitudes and speeds, reaching over 4,500 miles per hour and flying at altitudes above 100 kilometers, which is considered the edge of space. The program involved several test flights, with notable pilots like Neil Armstrong and Chuck Yeager participating. The X-15 provided valuable data that contributed to the development of future spacecraft and our understanding of aerodynamics and space travel.