The First Fleet Replenishment Program of 1931 gave Japan special permission to build new light cruisers with 6-inch guns with a total weight of 50,955 tons. Construction of the new fleet began at the Kuri Naval Arsenal in Mogami. The original Mogami was equipped with 5 triple 15.5 cm gun turrets, 4 triple 61 cm torpedo tubes, 4 twin 12.7 cm gun mounts, protection against 8-in. shells, 4 paired 25-mm guns and 2 paired 13-mm machine guns. The cruiser accommodated all this equipment within the limit of displacement of 8,500 tons. When naval contracts were cancelled, the ship was equipped with a 20.3 cm gun turret, which made the ships heavy and unstable. The Mogami has undergone numerous enhancement modifications, such as body reinforcement, bridge resizing, and repair of cracks and body plating. The Moghams participated in the World War II invasion of Java in the Gulf of Bantam.