Deliveries of FM-2s began in November of 1943, with 310 being delivered by the Jan 1, 1944. They didn't deploy aboard carriers until late April of 1944 and were in combat by July of '44.
Grumman began development of the this fighter in September of 1942, with the XF4F-8 being flown on November 8, 1942. Further development was done by Eastern Aircraft after all F4F/FM production was transferred to them to free up factory space for the F6F-3 at Grumman.
My regards,
Widewing