DARPA has selected the next design of its VTOL X-Plane, a crazy UAV with 24 ducted fans, a 4,000-hp engine, and the ability to hit speeds up to 460 mph.

Successful VTOL aircraft like the Harrier Jump Jet, or the F-35B s are able to angle their main engine exhaust downwards to generate lift—but in the early VTOL years, the Army and Navy opted for ...