Just as Bell and NACA had been working together since late in WWII to develop a supersonic research aircraft in the form of the XS-1/X-1, so had Douglas Aircraft and the Navy been engaged on a transonic "X-plane", the D-558 Skystreak. As this aircraft was designed to operate in the transonic regime for relatively extended periods, jet power in the form of an Allison J35 would be used rather than a rocket engine as in the X-1.There were plans for a second phase aircraft that would have added a rocket motor, but these were dropped as the Skystreak program was scaled down, although the follow-on D-558-II Skyrocket would have such dual propulsion.
The first Skystreak flew in May 1947, and by late August of that year had set a pair of world speed records in rapid succession. The second aircraft had a short life, with an engine failure leading to a crash in May 1948, but the third Skystreak would continue flying until the summer of 1953.
Bibliography:
"How New Douglas Skystreak Will Probe the Transonic" Aviation May 1947. Includes a diagram of air pressures to be encountered on the aircraft structure in flight, and several photos of wing construction.
Rolf Stibbe "Restoring a Skystreak" Warbirds International July 2000
