Yeager broke Mach 1 flying the Bell X-1 (or XS-1 according to some sources) rocket plane on October 14, 1947. He actually was in a slight climb at the time to help him lose speed faster in case of an emergency. The X-1 was never intended to be an operational fighter although they did put conventional landing gear on it "just in case" they decided to develop it as an interceptor like the Me-163.

The YF-100 was the prototype for an operational jet fighter. It was (supposedly) the first jet fighter to exceed Mach 1 in level flight. Older flighters like the F-86 Sabre could exceed Mach 1 in a dive. According to one article I saw recently, the XF-86 might've broken Mach 1 before Yeager's flight.

A recently-published book and concurrent article in Air and Space magazine (published by the Smithsonian Institution) by former aviator and air historian Al Blackburn lends credibility to the claim that test pilot George Welch broke the sound barrier in a prototype F 86 jet fighter before Captain Yeager's flight. In his book, Blackburn states that Air Force officials wanted to ensure that Captain Yeager and the X1 broke the sound barrier first.



Unfortunately, Blackburn has little data upon which to rest his claim. The North American Aviation team that coordinated the XF 86's test flights did not have access to instruments that could accurately measure the XF 86's speed until November, 1947. But with Welch flying at the edge of the XF 86's performance envelope, it appears that the XF 86 broke the sound barrier on at least one occasion before Yeager's October 14th flight. The speed of the XF 86 was finally measured with precision on November 13th, and showed that the fighter was capable of exceeding the speed of sound in the same type of dives Welch performed weeks before Yeager and the X1 officially broke the sound barrier.

At least two other swept-wing jet fighters may have broken the sound barrier before Yeager. The ME 262 was a WWII Luftwaffe fighter that was capable of dives that appraoched the speed of sound. The Russian MIG 15 was also being tested in 1947. Tex Johnston's obituary was written by Robert McG. Thomas Jr., and appears on page A16. The Air and Space magazine article appears on page 79 of the November-December issue.



As for the MiG-15, everything I've read on it says it had a maximum safe dive speed of about Mach 0.9. Yeager pushed a captured MiG-15 right up to the limit during some test flights. It never broke Mach 1. I'm pretty sure the critical Mach for the Me-262 was well below Mach 1, too.

Boeing's website doesn't go quite as far as the DataQuality link:

Forerunner of the operational Sabre was the XF-86, first flown Oct. 1, 1947, by North American test pilot George Welch. A few months later Welch became the first pilot to fly the plane at Mach 1 (the speed of sound) in routine flight. Although technically rated as subsonic, the Sabre is no stranger to supersonic speeds.