The Nagara class were authorized under the 8-8 Fleet Program of 1917 - 1920.
As part of the 8-4 Plan / 8 - 6 Fleet Completion Program of 1918 the Isuzu, Nagara, and Natori were authorized under the 1919 - 1920 estimates. As part of the 8-8 Plan the Abukuma, Kiru, and Yura were authorized under the 1920 - 1921 estimates.
The Nagara class cruisers were used as flagships of destroyer and submarine squadrons.
The bridge was built well above the main deck to allow an aircraft to be stored under it.
Initially there was a flying off platform but it was eventually replaced by an aft mounted catapult.
In 1940 the torpedoes were upgraded from 21" to 24". The Nagara class were the first Japanese Navy ships to carry 24" / 61 cm 8th year Type torpedoes.
In 1943 four more 24" torpedo mounts, of four tubes each, were added. Two of the 5.5" guns and the 3" gun were replaced by two 5" DP guns and two 13 mm guns (in 1940).