[Editor's Note: Keith wrote these short essays on the behalf of Eric Bergerud, who posted a query about U.S. AA improvements on the Mahan Email List. Keith has been so kind to allow this material to be posted. I have edited it some, but the content remains true to the original.]

It seemed unsatisfying somehow to list numbers of 20mm guns in 1943 without reference to earlier and later periods and to other light AA weapons, so this response has expanded beyond Eric's immediate question into a survey of the evolution of antiaircraft armament numbers in most major U.S. warship types. This does not purport to cover every class or to give exact numbers for every ship at every stage of the war; there were always variations within classes.

This is posted in five parts. The main sources are Norman Friedman's design histories, and the U.S. section, also written by Friedman, of Conway's 1922-1946 volume. A few basics, which will be familiar to many: During World War I some U.S. ships were fitted with 3-inch antiaircraft guns. During the late twenties and thirties the 5in/25 and the .50 cal became the major antiaircraft weapons. In the mid-1930s the famous 5in/38 dual-purpose gun entered service; beginning with the Sims-class destroyers, it was mated with the advanced Mk 37 director. In 1940-1941 the King Board recommended major increases in antiaircraft armament, including the addition of quadruple 1.1in machine cannon (which had entered service in the mid-thirties, but had been produced in only limited numbers to date). Because of the limited availability of the 1.1, some ships received 3in/50s as an interim measure. Even before Pearl Harbor the inadequacy of the 1.1 was recognized, and we planned to replace the 1.1 and the .50 cal with the 40mm and 20mm. During the war shipboard AA armament increased steadily.

A final surge in AA gun numbers came in response to the kamikazes in 1945, especially in destroyers; in some ships, for example, the Navy went so far as to remove all torpedo tubes for the sake of two more twin 40s. Although the main subject is light AA, I have also given numbers for 5-inch guns. For the most part these did not change, with some notable exceptions: e.g. carrier Saratoga and many of the old battleships received 5in/38 DP guns in place of their 5in/25s; many destroyers lost one 5-inch mount in return for increased light AA or antisubmarine armament.

I hope this document is of some use as a short reference, but it covers only the most tangible aspect of the great wartime advance in naval antiaircraft defense. A more complete discussion would require an analysis of fire control, radar, tactics, and training, as well as such technical advances as the proximity fuze.
[Editor's Note: Good idea; will go on my "list of things to write"!!]