The X-15 was a rocket-powered, high speed test aircraft that developed in the 1960s. It was dropped from a B-52 bomber, then ignited its engines and flew up to the edges of space.
Some of the flights in the X-15 exceeded the 50 mile altitude criterion for the Air Force to award the X-15 pilots astronaut wings. However, only one flight exceeded the 100km limit (62 miles) in order to be considered an astronaut internationally. Most of the pilots in the X-15 program would go on to train as astronauts at NASA, including Neil Armstrong.