The Iowa-class battleships were the last class of battleships built by the U.S. Navy, and continued on active service (with modifications) through 1992, when Missouri and Wisconsin were retired after providing support to troops in Iraq during Desert Storm. In one memorable incident, Iraqi troops surrendered to an RQ-2 Pioneer drone from Wisconsin.
During the design process, there was considerable debate over whether the Iowas should be merely improved versions of the South Dakota-class fast battleships, relatively lightly-armored "cruiser killers", or something else completely; this was complicated by the fact that three separate bureaux within the Navy Department were working on the design and communicated poorly with each other, if at all. In the end, unshackled from the limits of the Washington Naval Treaty and benefitting from a redesigned 16"/50 caliber gun which fit in a smaller barbette, the Iowas were finally funded in the Second Vinson Act of 1938 and the first two ships of the class, Iowa and New Jersey were laid down in 1940. Missouri and Wisconsin followed in 1941, and the last two ships of the class, Illinois and Kentucky, in 1942.
More design changes were to follow over the 52-year lifespan of the class. The original 1.1-inch AA guns were replaced with 20mm and 40mm guns, and as the 20mm proved to be ineffective against kamikaze attacks, they in turn were replaced by 40mm guns. The original OS2U Kingfisher floatplanes used for spotting and search & rescue were replaced with helicopters during the Korean War, and as drones became available, these were added to supplement the helicopters.
During the reactivation of the Iowas in the 1980s, there were proposals to fit the battleships with AV-8 Harrier jets, or possibly a flight deck suitable for the F/A-18 Hornet, but in the end the Navy simply upgraded the boilers, installed the Phalanx CIWS system, added electronic warfare suites, and also installed launchers for Tomahawk cruise missiles, Harpoon anti-ship missiles, and Stinger air defense missiles. It is alleged that W23 nuclear weapons (modified versions of the W19 nuclear artillery shell) as well as TLAM-A Tomahawk cruise missiles with nuclear warheads were carried aboard the Iowas, but Navy policy has been to neither confirm or deny these allegations.
As for Illinois and Kentucky, neither of these ships were completed; there were proposals to convert them into aircraft carriers or missile battleships, but none of these proposals were followed through on for reasons of cost - the Navy had plenty of carriers left over from World War 2, and the state of the art in missiles was advancing rapidly enough that by the time the conversions would be complete, they would already be obsolete. Kentucky was scrapped in 1959 after her bow had been used to repair Wisconsin; Illinois had gone to the scrapyard the previous year.