The Grumman F4F Wildcat was the standard carrier-based fighter of the United States Navy for the first year and a half of World War II. An improved version built by General Motors (the General Motors FM Wildcat) remained in service throughout the war, on escort carriers where newer, larger and heavier fighters could not be used. The Wildcat was outperformed by the Mitsubishi Zero, its major opponent in the early part of the Pacific Theater, but held its own by absorbing far more damage and wielding more firepower. With relatively heavy armor and self-sealing fuel tanks, the Grumman airframe could survive far more than its lightweight, unarmored Japanese rival. One Japanese Zero pilot reportedly emptied out his magazines trying to bring down a Wildcat during the Battle of Midway, only to see the Wildcat survive.