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 flight occurred on October 14, 1947, with 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 achievement was classified as top secret and the Air Force would not confirm the supersonic flight until March 1948.
October 15, 2012 12:46 PM PDT
Photo by: NASA Photo / USAF photo by Lt. Robert A. Hoover