In the last months of the Second World War, one question haunted aeronautics: what happens at the speed of sound? Pilots diving high-performance fighters had run into a wall of violent buffeting and vanishing control — the so-called sound barrier — and some had died trying to push through it. In 1944 the US Army Air Forces, el National Advisory Committee for Aeronautics (NACA) y el Bell Aircraft Corporation of Buffalo agreed to build an aircraft to find out. The contract for three aircraft was signed on 16 March 1945. It answered to no military or commercial mission; it existed only to fly into the unknown transonic region and come back with data — effectively inventing the dedicated research aircraft.

Bell’s answer was a small, straight-winged rocket plane painted international orange, its fuselage shaped like a .50-caliber machine-gun bullet — a form already known to stay stable faster than sound. Because a rocket burns its propellant in minutes, the X-1 could not take off with enough fuel to climb and still make a supersonic run, so it was carried aloft in the bomb bay of a modified B-29 Superfortress and dropped at around 25,000 feet, lighting its Reaction Motors rocket only once clear of the mother ship.

On 14 October 1947, US Army Air Forces Captain Chuck Yeager was dropped over the Mojave at Muroc Army Air Field — later Edwards Air Force Base — fired the rocket, and pushed the aircraft he had named “Glamorous Glennis,” after his wife, to about Mach 1.06 at roughly 43,000 feet. For the first time a piloted aircraft had flown faster than sound and returned. On the desert floor, observers heard what sounded like distant thunder — one of the first sonic booms ever made by an aircraft.

The achievement was classified. It leaked to the press in December 1947 and was only officially confirmed in 1948 — the same year the team behind it (Bell, Yeager and NACA’s John Stack) shared the Collier Trophy. What made the flight possible mattered as much as the flight itself: as shock waves rendered the conventional elevator useless near Mach 0.92, Yeager kept pitch control using the X-1’s all-moving horizontal stabilizer, a trimming tailplane that soon became standard on every supersonic fighter that followed.

The X-1 was only the beginning. Later versions pushed further and faster: on 12 December 1953 Yeager took the boosted X-1A to about Mach 2.44 before it tumbled out of control in a then-unknown phenomenon called inertia coupling, falling tens of thousands of feet before he recovered. Other X-1s set altitude records; several were lost to violent liquid-oxygen explosions later traced to a leather-gasket problem. The type retired in 1958 — but it had already launched the entire American X-plane dynasty that led, through the X-15, to the edge of space. Two of the original aircraft survive: “Glamorous Glennis” hangs in the Smithsonian, and the rebuilt X-1E stands at NASA Armstrong.