It's also largely PR guff. The first privately-developed supersonic aircraft was the Northrop N-156F, forerunner of the F-5, that first flew in 1959. Funded entirely from company funds with no military contract. And it went supersonic in its first flight with no drama.

In fact the chase plane for the Boom XB-1 is a T-38, derived from the N-156F. It can outrun the XB-1.