By the start of 1942 Grumman, the designers of the Wildcat, were ready to start production of the F6F-3 Hellcat, a much improved aircraft. However, the company did not have the production capacity to both maintain the required flow of Wildcats and produce the newer aircraft.
Fortunately, General Motors had several major east coast factories sitting idle, having decided to suspend commercial automobile manufacture during the war. GM created a new Eastern Aircraft Division with the intention of producing spare parts for existing aircraft.
However, Grumman and the U.S. Navy had a better use for that capacity. On 18 April 1942, Eastern Aircraft was given a contract to produce 1,800 FM-1 Wildcats (and another contract to produce the Grumman TBF Avenger, as the TBM). In the event they only produced 839 for the US Navy and 311 for the Royal Navy (where they were know as the Martlet V until January 1944 when the name was changed to Wildcat V). The first General Motors Wildcat flew on 31 August 1942. Over 800 FM-1s were produced during 1943, before production ended in favour of the FM-2.
The FM-1 was understandably very similar to the F4F-4. It had the same folding wing, used a 1200 HP Pratt & Whitney R-1830-86 air cooled radial engine. The only major change was that the six guns of the F4F-4 were reduced to four .50 calibre Browning M2 machine guns. The ammunition per gun was increased from 240 rounds in the F4F-4 up to 430 rounds in the FM-1. This was done in response to requests from navy pilots, who preferred the longer firing time provided by the four gun configuration, (originally used in the F4F-3).