The Mogami-class Heavy Cruiser were preceded by the Takao-class Heavy Cruiser and were designed within the restrictions of the Washington Naval Treaty. After Japan refused to recognize the treaty, however, they were modernized with heavier armor and greater armaments, which included swapping her original triple 155-mm turrets with twin 203-mm turrets; these swaps were possible because the Japanese Navy designed the Mogami-class ships specifically with this potential in mind.
The Mogami-class have been seen by naval architects as a design failure. The IJN's Naval staff insisted that each new class be superior to anything else in its category, yet designers strove to stay in compliance with treaty regulations. As a result, the initial construction of these ships was overly light; within their first few years of service, all four had to be reconstructed to remain seaworthy. They were also unstable seaboats due to excessive top-weight and their welded seams cracked under the stress of firing their own main guns.
While the Mogami-class ships were generally not considered to be a major improvement from the predecessor Takao-class ships, they were nevertheless used heavily during the Pacific War, and the experience gained from the Mogami-class design at least partially contributed to the subsequent successful Tone-class design.