I've seen photos of an OD B-29 in England during 1944, but AFAIK it was a counterintel stunt to convince the Germans that it was there to train the pilots and crews for full scale deployment of the type to the 8th AAF. Flew around, made a couple of appropriately hush-hush visits (as hush hush as a B-29 can get, which was the point) and then was flown either back to the states or to the CBI.