X-1 hits Mach 1

The history of X-Planes begins with the X-1. It wasn't just the first in the lineage--it was the first aircraft ever to break the sound barrier.

That famous flight took place on October 14, 1947, with the legendary Chuck Yeager in the cockpit. The photo here shows the Bell Aircraft X-1-1 in flight, along with a snippet of the paper tape (which tracked the flight data) showing the jump to supersonic speed at Mach 1. (The first glide flight had happened in January 1946.)

NASA points out that the exhaust plume here shows the shock wave pattern. The achievement was classified as top secret; the Air Force would not confirm the supersonic flight until March 1948.