Grumman X-29: The impossible fighter jet with inverted wings

There's no airplane quite like the Grumman X-29. Its astonishing forward-swept wings were just one of its many bold innovations.

Created at the height of the Cold War by a conglomerate of giants -- NASA, the US Air Force, the "men in black" at the Defense Advanced Research Projects Agency (DARPA) and aerospace behemoth Grumman -- it first flew in 1984 as part of a quest to build the ultimate fighter jet.

But its highly experimental design made it the most aerodynamically unstable aircraft ever built.

"It was unflyable -- literally -- without a digital flight computer on board, which made corrections to the flight path 40 times a second," said Christian Gelzer, chief historian at the NASA Armstrong Flight Research Center in southern California (where the plane was tested) in a phone interview.

"The engineers concluded that if all three flight computers had failed together, the airplane would have broken up around the pilot before the pilot had a chance to eject."