In the world of aviation, X-Planes hold a special place. They're a glimpse into the future, a look at what's coming in a later generation of aircraft -- and, to a degree, spacecraft as well. Over the last seven decades, they've been a proving ground for developments including delta wings, tailless aircraft and supersonic flight.

Where once the experimental aircraft required the steady hand and quick reflexes of a test pilot in the cockpit, in recent years, they've tended to be unmanned vessels -- a central theme in aerospace advances generally. Those newer, pilotless X-Planes include the X-37B orbital vehicle and the hypersonic X-51A.

In this slideshow, we'll be taking a look back at X-Planes over the years, starting with this vintage group shot from 1953. In the center is the X-3, and clockwise from the left are an X-1A, a D-558-1, an XF-92A, an X-5, a D-558-2 and an X-4. (A few aircraft, such as the D-558 series and the later M2-F1/2/3 series, never got the "X" designation, though they're included in the family because they served the same goal of flight research.) We'll also look at what the future might hold.

Originally published May 27, 2010.
Updated April 3, 2018: Added NASA's Low-Boom Flight Demonstrator and information on other recent developments.