Like a lot of countries, Atlantis took their last large light cruiser, the Triton Class, and turned the same hull, propulsion and superstructure into a heavy cruiser, armed with 12x7.5". Atlantis kept the size of the ship to reasonable dimensions by not fitting seaplane handling equipment. This saved 50-60 feet of hull size and several thousand tons of displacement. Heavier armour was fitted with a 5.5" armoured belt and 3" of deck armour. At 611 feet long with a breadth of 68 feet, the ship required 80,000shp to move it along at 32 knots.
The final completed armament was, 12x7.5", 14x4", 32x40mm, 6x21" torpedo tubes.
Originally to have been a class of four, only the first two laid down in 1939 and completed in 1942 made it to the war as cruisers, the other two ships were postponed on the outbreak of war, but 6 months later were restarted to a new design, the two ships were to be completed as Light Aircraft Carriers. (Guess what drawings coming next?)