Anti-Air Armament Development of U.S. Cruisers
by Keith E. Allen

Omaha class
These old light cruisers were completed in the 1920s with an AA battery of four 3in/50. The first .50 cals were fitted in 1933. The King Board planned to give them four 1.1s quads. I'm not sure how extensively this was implemented; some at least got more 3-inchers as an interim measure. There were even plans to turn these ships into antiaircraft cruisers, like some of the old British light cruisers. By 1944 most of these ships had six 3in/50, three twin 40mm, and twelve 20mm.

Treaty Cruisers
This rubric covers all the U.S. cruisers built in the late twenties and thirties under Treaty restrictions: the Pensacola, Northampton, Portland, New Orleans, and Brooklyn classes (including their half-sisters, Helena and St. Louis), and the one-of-a-kind Wichita. The Pensacola and Portland classes as built had a main AA battery of four 5in/25. Beginning in 1938, this was doubled to match the other cruisers; I'm not sure whether they all got this upgrade. The New Orleans and Brooklyn classes were completed with eight 5in/25. St. Louis, Helena, and Wichita were the first cruisers with 5in/38s. The standard light AA battery of all cruisers by 1940 was eight .50 cals. This was original equipment beginning in the Portland class in th early thirties. In 1940-1941 the King Board called for four quad 1.1s on each cruiser; as with other types, many ships initially received 3-inch guns instead. At least by early 1942, some cruisers had 20mm. By January 1942 the standard 20mm battery for large cruisers (i.e. all except the Omaha and Atlanta classes) was set at twelve guns, and by January 1943 it was raised to twenty; of course in practice these numbers varied.

A few figures on specific classes follow. By December 1941 the Pensacolas and Northamptons each had either four quad 1.1s or four 3in/50s. By 1944 Pensacola and her sister Salt Lake City had six quad 40s and 20 to 21-20mm. The Northamptons had about the same armament, although it varied from ship to ship. Portland at the end of the war had 24-40mm (four quad, four twin) and 12-20mm.

Chuck Hansen, in "USS San Francisco: A Technical History," gives these details on the evolution of her AA armament, which seems to be typical of the New Orleans class:
---Completed with eight 5in/25, eight .50 cal. ---In 1940 four 3in/50 were fitted pending the availability of 1.1s. ---By 7 December 1941 her 3-inchers had been removed at Pearl. On the evening of 7 December her four 1.1 quads were installed. ---In March 1942 all .50 cal were removed and replaced by twelve 20mm. After February 1943, San Francisco had twenty 20mm. At this time the 1.1s were removed in favor of four quad 40mm. ---In October 1944 she received two new quad 40mm, for a total six quads, and a new 20mm armament of 28 Mk 10, in place of her twenty Mk 4.

The large light cruisers of the Brooklyn class had about the same AA armament as the heavy cruisers. In early 1942 they exchanged their interim 3in/50s and .50 cals for 1.1s and 20mm. By the end of 1942 Helena purportedly had four quad 40s and 12-20mm, which seems heavy for that stage of the war (from the Leeward Battle Damage Report on Helena's loss). At the end of the war most of these ships had 28-40mm (four quad, six twin) and 18 to 20-20mm. Savannah received 5in/38s during refit after she was hit by a glide bomb at Salerno. Wichita had eight single 5in/38s throughout the war. She ended the war with 20-40mm and 18-20mm.

The original armament of the Atlanta-class antiaircraft cruisers was sixteen 5in/38, four quad 1.1s (the first two ships, Atlanta and Juneau, had only three when completed, but later got the fourth), and eight 20mm. In 1943 the two survivors of the first group, San Juan and San Diego, received 40mm in place of the 1.1s. At the end of the war San Juan had 14-40mm (one quad, five twins) and nine 20mm. The second group of this class were built without the two waist 5-inch mounts, for a total of twelve 5in/38. They had eight twin 40mm and sixteen 20mm; one would have thought the deletion of two twin 5-inch mounts would have allowed for a greater increase than that in light AA.

Large Wartime Cruisers
The Cleveland-class light cruisers began commissioning in mid-1942. They had a secondary battery of twelve 5in/38. The standard light AA armament at first was 12-40mm (two quad, two twin); I'm not sure how many 20mm were fitted at this point. By May 1944 the standard battery was set at 28-40mm, although there were some variations in practice. They had between 13 and 28-20mm.
The Baltimore-class heavy cruisers carried 48-40mm and 24 to 28-20mm.
The Alaska-class large cruisers had fourteen quad 40s and 34-20mm.