From experience gained with the T-Class submarines the US Navy decided to build a series of 6 boats (SS 212-217) as part of the 1940 program, to be followed by a series of 67 boats (SS 218-284). This class was called "Gato", named for the first boat in the class. (This has since been the naming procedure for all classes.) They were to be the same size and armament as the T-Class, but with different propulsion gear arrangements and some differences in other structural and external details.
All boats of the class had heavy structural mountings for deck guns, one forward and one aft of the conning tower on the main deck. Generally only one gun was mounted, but it was by no means rare to see boats fitted with two. (The choice of armament was generally left up to the commanding officer of each boat though availability played a big part.) There were also usually two anti-aircraft cannons in varying combinations mounted fore and aft on the bridge deck.
The introduction of heavy anti-aircraft guns made it necessary to make many modifications to the conning towers. Initially, the towers were quite small, but as the number and caliber of the weapons increased the tower size also increased. Normally there were two periscopes of considerable length and size. Three periscopes were fitted in some boats.
The Gato Class were very successful boats. Because they were fast, strong, and had a long range, they could take long offensive cruises in the Pacific. They were self-sufficient, not needing supply boats such as required by the German U-boats.
For some years after the war, and until the commissioning of nuclear and missile-launching submarines, the survivors of this class along with the later Balao (SS 285-416) and Tench (417-522) classes formed the backbone of the US Navy's submarine fleet.