January 9, 2014 2:24 PM EST

The Pentagon figured out during the Vietnam era that it could build a better warplane by giving it wings that swept back and forth in flight. With the wings swept forward, the planes could carry more fuel and weapons. With the wings swept back, they could fly faster during attack runs.

But that flexibility comes at a cost, as a recently released Air Force probe into the crash of a swing-wing B-1 bomber last August in Montana makes clear. As the four-man crew prepared for a practice bombing run–their first since returning from missions over Afghanistan–the left wing pushed a dislocated seal into a fuel line. A thousand gallons of jet fuel quickly flowed out, triggering a series of blasts that blew the plane apart as–amazingly–the crew safely ejected.

Better, computer-aided designs long ago led to warplanes that don’t need movable wings to maximize lift and minimize drag. Which means the B-1 is likely to be the last swing-wing aircraft flown by the U.S. military.

This appears in the January 20, 2014 issue of TIME.

Contact us at editors@time.com.

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