Fort Varnum was built as part of a general modernization of US coast defenses begun in 1940 with the outbreak of war in Europe and the Fall of France.

It was built to relocate previously-emplaced weapons to a more useful location nearer the entrance to Narragansett Bay.

Fort Varnum's main armament was Battery House, two 6-inch M1900 guns on pedestal mounts, completed in 1942.

[1][2] Two 3-inch M1903 guns on pedestal mounts were planned for Battery Armistead, relocated from Fort Kearny, now the University of Rhode Island Narragansett Bay Campus.

[1] Several fire control stations were built in Narragansett disguised as beach cottages; these may date from the start of the Endicott Program circa 1900.