Tread lightly about what? DARPA's job is to explore the seemingly ridiculous. If you had any idea some of the stuff that's now "normal" that started from a DARPA program, you'd be floored. As for airplanes that stay aloft for long durations, NASA was in the process of doing just what DARPA is looking for with the Pathfinder and are hoping to extend that with the Pathfinder Plus. NASA had Pathfinder aloft for several weeks before it flew into a storm and was destroyed.