like the Federation of American Scientists and the Air Force Museum websites, the MiG-19 first flew in September of 1953. It didn't say when it actually broke Mach 1 for the first time. I've seen a few websites that say the MiG-19's first flight was on May 25, 1952 but I can't vouch for their credibility.

The old Soviet Union was very secretive during those days, so it's hard to tell when they first exceeded Mach 1 in level flight. It's possible that some non-production prototype aircraft went supersonic before the MiG-19 or YF-100. There was a MiG prototype supersonic fighter with the designation I-350. It seems to have first flown in 1951 but was cancelled after only 5 flights due to trouble with the engine. I don't know if it ever went supersonic but I kind of doubt it. It was reengined to become the I-360, which became the prototype for the MiG-19. Some sources say the I-360 first flew in 1952. If accurate, then perhaps it broke Mach 1 in level flight before the YF-100.

The Air Force Museum states that the first flight of the YF-100 was on May 25, 1953. It actually broke Mach 1 on its maiden flight.

As for the YF-100, Yeager describes how the prototype had too little vertical tail area making the plane dangerous. A North American test pilot, George Welsh*, was killed flying an early production F-100. The planes were modified to have greater vertical stabilizer area.

* Welsh was one of a handful of pilots to successfully take off and engage the Japanese during the Pearl Harbor attack.

Oh, and a very Happy New Year to you, too.