X-Wing

Early/mid 80’s artwork depicting an X-Wing vehicle (a helicopter with very rigid rotors that could stop in flight to serve as wings). The text attributes this to Boeing-Vertol, but I could swear I’ve seen it attributed to someone else (Lockheed? Grumman?).

The X-Wing concept seemed to have great promise, but testing showed that, perhaps unsurprisingly, it had severe problems during transition. Had it worked, though, it would hve provided the high speed and long range of a conventional aircraft with the hovering efficiency of a helicopter.

Source: http://www.aerospaceprojectsreview.com/blog/?p=4731