 At its introduction into service in 1962, Minuteman I was fitted with the W59 warhead with a yield of 1 Mt.Production for the W56 warhead with a 1.2 Mt yield began in March 1963 and W59 production was ended in July 1963 with a production run of only 150 warheads before being retired in June 1969.The W56 would continue production until May 1969 with a production run of 1000 warheads.