There is unsubstantiated rumor that a Me-262 went supersonic towards the end of WW II.
There's quite a bit on early - probably inadvertent and unrecorded - supersonic excursions in various types in Winkle Brown's "Wings on my Sleeve" - which you should read anyway!
Winkle was a great proponent of the Miles M52 (an early political cancellation?) which would have been a likely contender to beat Chuck to it - but it didn't!!
He (Winkle) wrote a book on the M52, which I'm about to start reading - when I finish re-reading "Wings"!