During 1961 and 1962, construction began on 12 SM-65 Atlas missile sites. The sites were built within a radius of the base and were completed in 1963. The squadron was equipped with 13 missiles, allowing each silo to have its Atlas missile with one left for spare. When one missile in a silo was scheduled for maintenance, the spare missile was sent to replace it. This allowed the 380th to maintain 12 ready to launch missiles seven days a week. All sites were in New York state except for two located on the other side of Lake Champlain in Vermont. The 556th Strategic Missile Squadron became completely operational on 15 September 1964. This was the last Atlas squadron to be accepted and the only Intercontinental ballistic missile base east of the Mississippi River. The 556th's last operational day was 30 April 1965 with the squadron's inactivation on 25 June 1965.