The R-class submarines were a class of United States Navy submarines active from 1918 until 1945. The R-1 thru R-20 boats built by Fore River Shipyard and Union Iron Works were known as the "R-1 class" subs. These single-hull submarines were structurally very similar to the preceding O-class, but with the addition of 21-inch torpedo tubes and a fixed (rather than retractable) gun mount.