In 1935, while its F3F biplane fighter was in the initial stages of testing, Grumman Aircraft Engineering Corporation began work on a prototype F4F, engaging in a competition with Brewster Aeronautical Corporation to provide the Navy's next carrier-based fighter. The F4F Wildcat contrasted sharply with its primary adversary in the Pacific, the Japanese Zero. While the former possessed unmatched maneuverability, rate of climb, speed and range, the F4F's strength rested in its ruggedness and firepower. Grumman discontinued the Wildcat in favor of the Hellcat in 1943, but the Eastern Aircraft Division of General Motors continued production of the design under subcontract. With a more powerful Wright R-1820-56 Cyclone engine, General Motors built Wildcats carried the designation FM-1/2 and operated primarily from escort carriers.