The first USS Tunxis was launched on 4 June 1864 at Chester, Pennsylvania, by Reaney, Son & Archbold; and commissioned at the Philadelphia Navy Yard on 12 July 1864.

Though the original designs for the Casco-class monitors were drawn by John Ericsson, the final revision was created by Chief Engineer Alban C. Stimers following Rear Admiral Samuel F. Du Pont's failed bombardment of Fort Sumter in 1863.

It was discovered that Stimers had failed to compensate for the armour his revisions added to the original plan and this resulted in excessive stress on the wooden hull frames and a freeboard of only 3 inches.

He was forced to raise the hulls of the monitors under construction by nearly two feet and the first few completed vessels had their turrets removed and a single pivot-mount 11 inch Dahlgren cannon mounted.

On 19 October 1864 Tunxis entered William Cramp & Sons' shipyard, Philadelphia, for extensive refit and rebuilding.