GAU-8

Lifted from several Google sorces:

Hans Guido Mutke claimed to have broken the sound barrier on April 9, 1945 in a Messerschmitt Me 262. However, this claim is disputed by most experts as the Me262's stucture could not support high transonic, let alone supersonic, flight and lacks a scientific foundation.

George Welch apparently broke the sound barrier on October 1, 1947 while diving the subsonic XP-86 Sabre. 13 days, 30 minutes before Yeager's historic flight. Welch apparently repeated his supersonic flight. Although evidence from witnesses and instruments strongly imply that Welch achieved supersonic speed, the flights were not properly monitored and cannot be officially recognized. (The XP-86 officially achieved supersonic speed on April 26, 1948.)

Chuck Yeager (then a Captain in the US Air Force) is the first person accepted and documented to have broken the sound barrier in level flight on October 14, 1947, flying the experimental Bell X-1 rocket plane to Mach 1 at an altitude of 45,000 feet. Yeager's flight was part of a test program with the goal of achieving supersonic flight so proper monitoring was in place for the flight.