Bell X-1
1947
The small orange rocket plane that broke the wall everyone else was still describing as impossible: on 14 October 1947 Chuck Yeager flew it past Mach 1 in level flight - the first crewed aircraft to beat the speed of sound, a rumour the press had been calling "the sound barrier" as if it were solid.
Specs
Launched from the bomb bay of a B-29 at 7 km, because no engine of the day could get it airborne under its own power; four chambers of liquid rocket fuel burning in sequence, the big one for roughly the time it takes to boil an egg; all-moving tailplane - the invention that mattered, because conventional elevators went rigid and useless as the shock waves rolled over them. Shaped like a .50-calibre bullet, deliberately: it was the only known supersonic body. Yeager flew the historic run with two broken ribs, riding the galley stairs to the bomb bay because he could not twist to fit the harness.
Why it matters
The "barrier" was never a wall - it was a region where the old control surfaces stopped working and nobody yet knew what to build instead. X-1 proved the air did not end at Mach 1; it just changes its rules. Every military aircraft and every launch-vehicle airframe designed since has been drafted against the data that small rocket plane collected, including the escape tower philosophy and re-entry control that made crewed spaceflight possible. The lesson generalises past aviation: the thing called impossible is usually a missing component in an unfinished model.