Jewell Island was part of the Harbor Defenses of Portland fort network.
The island is rich in legend—tales are told of Captain Kidd cruising its coast and hiding treasure.
It was designed for two 6-inch guns in shielded mounts that would be on the large concrete circles at each end of the battery.
Positions for two Anti-Motor Torpedo Boat (AMTB) batteries with 90 mm guns were also constructed on Jewell Island.
The World War II construction was part of a modernization of the Harbor Defenses of Portland, centered on Battery Steele on Peaks Island, which the fire control towers and SCR-296 radar on Jewell Island supported.