During the design process of the , which began immediately after the treaty was signed, the US Navy became aware that the next class of Japanese cruisers, the , would be armed with a main battery of fifteen 6-inch guns, prompting them to adopt the same number of guns for the "Brooklyn"s. After building seven ships to the original design, additional changes were incorporated, particularly to the propulsion machinery and the secondary battery, resulting in the "St. Louis" sub-class, of which "Helena" was the second member.