Intelligence reports in 1916 suggested that the Germans were going to build new cruisers with 15 cm / 5.9" guns that would outclass the C classes of light cruisers. It was felt that just adding one gun would be more than enough. To make room for this 6th gun the hull was lengthened and the bridge moved back to fit it in front.
The beam of the Danae class had to be increased by 2' 9" due to the top weight increasing.
The Danae, Dauntless, and Dragon were ordered in September 1916.
The Delhi, Dunedin, and Durban were ordered in July 1917 / September 1917.
The Daedalus, Daring, Despatch, Diomede, Dryad, and Desperate were ordered in March 1918. The Daedalus, Daring, Dryad, and Desperate were cancelled on November 1918.
Battle of Jutland Influences
The Battle of Jutland happened during construction and it was decided to increase armor around the magazines and provide better protection for the main guns. Dual tube torpedo launchers were increased to three and depth charge launchers were added.
The Dauntless and Dragon had hangers placed under the bridge. Delhi, Dunedin, Durban, Despatch, and Diomede initially had fly off platforms and these were to later be replaced by catapults but never were. They never carried aircraft and all the equipment was removed by the end of the 1930s. Danae did not carry aircraft.