The two were the last heavy cruisers completed for the Imperial Japanese Navy. The Tone-class cruisers were originally envisaged as the 5th and 6th vessels in the Mogami-class. However, by the time construction began, serious weaknesses in the Mogami-class hull design had become clear following the Fourth Fleet Incident in 1935. As Japan no longer was obligated to abide by the limitations of the London Naval Treaty, a new design was created and new means of construction were utilized. Though the external dimensions were close to the Mogami-class, the design was quite different, with all the main battery of guns placed forward of the bridge, reserving the entire stern area as a large seaplane hangar. Unlike the U.S. Navy, the Japanese did not have a dual role attack/scout aircraft, nor did