The X15 was built by North American, and I do not think Yeager flew it. You may be thinking of an ill-conceived idea for Yeager to use one of the NF-104A rocket-boosted Starfighters to set an altitude record. According to this series of articles by the pilot running the NF-104A test program [1], Yeager did not follow the flight profile and consequently ended up in an irrecoverable flat spin from which he had to eject. Yeager could not be blamed, so the author claims he got shafted by the inquiry, with no opportunity to defend himself.

[1] http://www.kalimera.org/nf104/stories/stories_11.html http://www.kalimera.org/nf104/stories/stories_12.html http://www.kalimera.org/nf104/stories/stories_13.html