Grumman TBF Avenger was a torpedo bomber developed initially for the United States Navy and Marine Corps, and eventually used by several air or naval arms around the world. It entered U.S. service in 1942, and first saw action during the Battle of Midway. Grumman's first torpedo bomber was the heaviest single-engine aircraft of World War II, and only the USAAF's P-47 Thunderbolt came close to equalling it in maximum loaded weight among all single-engined fighters, only being some 181 kg lighter than the TBF, by the end of World War II. The Avenger was the first design to feature a new wing-folding mechanism created by Grumman, intended to maximize storage space on an aircraft carrier; the F4F-4 and later models of Wildcat received a similar folding wing and the F6F Hellcat employed this mechanism as well. The engine used was the Wright R-2600-20. There were three crew members: pilot, turret gunner and radioman/bombardier/ventral gunner.