From what I've read, Lockheed-Martin has been working on a proof of concept vehicle since 2013 and it's intended to be semi-autonomous.
It reportedly has a "dual mode" scramjet and turbine combo engine and *should* be capable of over 4,600mph (as with the SR-71, the world may never know TAS).
It's most likely that any first flight flights have already happened - Lockheed is pretty good about being sneaky.