The principle of the fuse was the ability of radar to locate a target. Micro-radar sets were made that were capable of being fired fitted in a shell and that sent out signals during the shell's flight. When the response showed that the shell was within killing range of the target, an electronic signal detonated the shell.
During World War II, British and American scientific research successfully combined to produce a new method of shell fusing that enabled the shell to sense its target electronically and to explode when it was within lethal range. The British began work on this in 1939, and by the summer of 1940, the United States was involved as well. The first work was carried out by the National Defense Research Committee (Section T). Other participants included the Department of Terrestrial Magnetism of the Carnegie Institute, the Sylvania Company, RCA, the University of Michigan, Johns Hopkins University, the National Carbon Company, Raytheon, and Eastman Kodak. At the end of the war, the Naval Ordnance Laboratory took over research and development, producing infrared proximity fuse systems.
Shells with proximity fuses were in production by early 1942. The Americans knew it as the VT fuse (VT was a code name and did not stand for "variable time" or anything else). The first successful action for the proximity fuse came in January 1943 when a 5-inch gun in the secondary battery of the U.S. cruiser Helena shot down an attacking Japanese bomber. Initially, use of the proximity fuse was limited for security reasons to oversea actions, and the fuses were producing spectacular results by the time the Japanese mounted their suicidal kamikaze attacks. The fuse was also used in ground operations later in the war, and it was particularly effective against German troops in the Battle of the Bulge. It was also very important in the gunnery battle against the German V-1 buzz bomb.
The proximity fuse was developed in total secrecy. It was perfected by the time it was issued, and it worked. Its successful development and procurement is a rare accomplishment in the history of weapons development.
Baldwin, Ralph B. The Deadly Fuse. London: Jane's Publishing, 1980.; Hogg, Ian V. The Illustrated Encyclopedia of Artillery. London: Hutchinson, 1987.