We recently discussed our favorite vaporware cars, but I would argue that vaporware planes, helicopters and other flying machines are even weirder and more interesting.

Take the Boeing 2707, aka Boeing SST, for example. It was the first American supersonic plane project that started in the late 1950s and featured a number of fascinating technical innovations. But it was doomed by questions about its economic viability, concern over sonic booms, and ultimately a lack of government funding.

Only two incomplete prototypes were ever made. The project did, however, give Seattle's NBA team their name.

That's our question of the day: What's the craziest failed aircraft project? And by "failed," we mean something that didn't meet its intended goals in some way.