Grumman Aviation produced the G-18, the prototype XF4F-2, which first flew in September 1937. Initial evaluation revealed several performance problems, which led the U.S. Navy to reject the G-18 in favor of the Brewster's F2A "Buffalo". The Navy asked Grumman to continue with the project, and resulted in the G-36, which flew in February 1939. The G-36 tests exceeded the performance of the F2A, resulting in the U.S. Navy approving the design and ordering it into production as the F4F-3 Wildcat. Two yeas following its deployment, during the critical stages of the Pacific War in 1942, the improved F4F-4 Wildcat was introduced. It had folding wings, and three 12.7mm machine guns in each wing, plus the more powerful 1,200 P & W R-1830-86 Twin Wasp engine, giving it a speed of 512km/h. Grumman became known as the "Ironworks" due to its strongly designed aircraft, which could absorb enormous punishment and still fly home.