1. Lockheed M-21 Blackbird —
The world of aviation has been one of constant experimentation. Sometimes, alas, the coolest models don't takeoff, or are retired in their prime.
Here we take a look at ten planes which, for better or worse, have been retired.
Dan Hagedorn, curator of Seattle's Museum of Flight, points to the now-defunct Lockheed M-21 Blackbird as one of history's greatest flying machines. The spy plane built to support a CIA program in 1963. It was a precursor to the SR-71 Blackbird, which Hagedorn describes as "the fastest, highest-flying piloted jet in history."