Just prior to the start of the war in the Pacific, the United States Navy in 1940 was once again in the process of upgrading its' fleet submarines. An ambitious plan called for the construction of 73 Gato class boats.

Using the Tambor Class submarine as the model standard, the Gatos had incorporated numerous improvements which increased its' overall patrolling and combat capabilities. The modifications made to the diesel engines and batteries increased the Gato's patrol duration over that of the T-Class subs and internal alterations provided more amenities for the crew. The Balao and Corsair - Tench Classes were virtually identical to the Gatos, with just slight internal differences. The Balaos were the first of the "thick skinned" subs. Higher strength steel used in the pressure hull increased their test depth to 400 feet.  

As the war progressed, modifications to the tower and equipment were commonplace. Removing much of the plating on the periscope shears and near the cigarette decks reduced the boat's silhouette and decreased the time it took to submerge. Due to the many alterations at the different shipyards, no two boats looked alike at the wars' end.  Hull colors changed during 1943 (?). In an effort to reduce their detection from both surface ships and aircraft, the original "lamp black" color was changed to "ghost gray" (submarines at periscope depth were often visible from the air in a black hulled boat). Even the periscope tips were painted "pink", many believing that this color took on the hue of surrounding area.