The Mogami class fell under the London Naval Treaty of 1930 / Washington Naval Treaty. Construction was approved under the 1931 Supplementary Program. They were designed to carry the largest guns (6.1" allowed) and yet still fall under the treaty restrictions. Electric welding and light alloys were used to get them under treaty restrictions. It was planned to upgrade to 8" guns when the treaty restrictions expired.
During trials it was found that the electric welding was faulty and the ships proved to not be a stable platform for firing. In a 1936 refit the weak welds were strengthened.
There were two catapults for the aircraft.
The 6.1" turrets that were removed were later used in the Yamato class battleships.
The engine rooms exhaust were trunked into a single funnel.
In 1937 they had their hulls bulged as it was found that they were rather vulnerable due to their slimness.
In 1939 the 6.1" turrets were replaced by 8" turrets.