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. It's those newer, pilotless X-Planes that have been in the headlines of late, first with the X-37B that was launched into orbit last year, and then again with 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.)