I know Forts had a chemical toilet, but a bathtub? You'd have to really heat up the water..and I wouldn't want to strip down at 25k feet in an unheated airplane to get into the thing. Plus, since the B-17 is a taildragger, you'd have to fill and drain the tub in flight to keep the water from sloshing out..maybe if it was mounted on some sort of gimbal to keep it level....
SN