Although not designated an "X vehicle," the D-558-2 was essentially an X-vehicle aircraft in design and function, and contributed much to aeronautics research. The D-558 Phase Two aircraft was quite different from its Phase One predecessor, the Skystreak. German wartime aeronautical research records, reviewed in 1945 by Douglas Aircraft Company personnel, pointed to many advantages gained from incorporating sweptback wing design into future research aircraft.

Credits - NASA