Beginning at the height of World War II, America joined several other western nations in preparing for high speed/supersonic flight utilizing the new propulsion engine, the jet.
Limitations as to thrust of the new engines negated their use in attempting to break the sound barrier and thus, liquid fueled rocket engines were substituted. These "rocket planes" were couched within the government program known as the X Series of aircraft.
The X-1 came first, piloted by the Chuck Yeager who officially broke the sound barrier in 1947. As each successive airplane was developed and flown, the reach for higher speeds was well underway.
By the late 1950s, the X-15 made its debut and promised the highest speed yet for manned aircraft: Mach six, or more.
After successful glide drops, the day for the first rocket powered flight approached and the chief test pilot for North American Aviation, builders of the X-15, Scott Crossfield took to skies, carried aloft under the wing of the B-52 mother ship on 17 September, 1959. Flying over the prescribed 480-mile test route from Nevada to the deserts of Southern California, Crossfield pushed the button and ignited the XLR-11 rocket engines, and history was made.
The X-15 went forward with the new NASA and flew 199 test/research missions in its decade-long career. Not only flying faster, it also flew higher, actually reaching the edge of space. Pilots who attained this altitude were awarded astronaut wings. Of the several pilots who flew the aircraft, two future astronauts made their own history flying the craft. One was Joe Engle, someday-Shuttle commander, and the other, a young, sharp Navy pilot named Neil Armstrong. Flying the varied research profiles brought not only invaluable experience to these two pilot-astronauts but also put them on the map for selection into the space program. Armstrong's great repute rewarded him the job of landing the Lunar Module on Tranquility Plain.