The DuPont DP-2 was (or perhaps still is) a concept for a VTOL jet transport, with a general configuration that hearkens back to the Bell X-14. It has been under development for an extremely long time, acruing a great deal of controversy. Read about that here, here and here. See images of it here.
The plane which certain parties claim “does not fly” did in fact achieve stable, controlled tethered hover in September 2006.
The military requirement for a successful test is a 30 second hover within a small virtual box. The DP-1 UAV, a 54% scale version of the larger DP-2, completed several autonomous flights of approximately 45 seconds at duPont’s El Cajon facility within a smaller envelope than the contract required.
Images and videos of the test flights can be seen at the following link:
http://www.dupontaero.com/DP-1/dp-1.html