The powerplants and propellers were supplied by the AAF. The problem wasn't counter-rotation itself, but the fact that it was poorly implemented. The propellers really weren't compatible with the engines, espescially in that arrangement.Yes, I read that on wiki
But what components did the Airforce supply to the prototype? And how can a counter-rotating propeller create a strong vibration on XB35? I don't remember Tupolev Bear has that problem (or is it?)
The real problem with flying wings is this question. What does it do better than conventional airplanes that we already build?
There's a sadly long gap between coolness and usefulness.