The Sikorsky-designed R-6A two-seat observation helicopter was a refined version of the R-4, the first Air Force helicopter to serve in a combat zone (May 1944). First flown in October 1943, the R-6A used the same rotor and transmission system as the R-4, but had a more powerful engine. The fuselage was completely redesigned to reduce air resistance and provide improved pilot visibility; therefore, the R-6A bore little resemblance to its boxy-looking predecessor. Sikorsky built the first six of the helicopters (one XR-6 and five XR-6As), but another 219 R-6As were produced in 1944 and 1945 by Nash-Kelvinator under a Sikorsky License. Although primarily an observation and liaison helicopter, many R-6As also were equipped with capsules on each side of the fuselage to carry litters for medical evacuation. Bomb racks also could be installed and, if necessary, the R-6A could be equipped with floats for operation from water.