Some early models of Helicopter did see service with the US & Germany during WW2 but only in limited numbers. Whether they'd have made a significant difference in combat is debatable. The technology wasn't developed enough at that time. The Helicopter really came in to it's own in the 60's especially in Vietnam.
use of helicopters would have made a great difference in at least one sphere as later wars proved. In the Korean war, M*A*S*H units used helicopters to ferry the injured from battlefields to medical units and beyond. Casualties were significantly reduced. Only in Vietnam, did the full scale of helicopter use become apparent.
They did see service to a limited extent as supply craft and for rescue operations in the China-Burma-India theater, and were operated by the 1st Air Commando unit. World War II demonstrated that the helicopter could perform useful missions, but the helicopters of the day were still limited in their power, size, and hence their capabilities. Here is a list of helicopters used in WW II from Wikipedia
