USS[a] Illinois (BB-65) was the fifth Iowa-class fast battleship that was laid down for the United States Navy during World War II in the 1940s, although she would not be completed.

Illinois was laid down in December 1942, but work was given a low priority, and was still under construction at the end of World War II.

The Iowa class of fast battleships was designed in the late 1930s in response to the US Navy's expectations for a future war with the Empire of Japan.

Design studies prepared during the development of the earlier North Carolina and South Dakota classes demonstrated the difficulty in resolving the desires of fleet officers with those of the planning staff in the displacement limits imposed by the Washington Naval Treaty system, which had governed capital ship construction since 1923.

[2][3] The last battleships to be built by the United States, the Iowa-class ships were also the US Navy's largest and fastest vessels of the type.

[2][4] Illinois differed from her earlier sisters in that her design called for an all-welded construction, which would have saved weight due to increased strength over a combination riveted/welded hull used on the four completed Iowa-class ships.

Tests with caissons in 1943 led to improvements for the torpedo defense system that increased its resilience to underwater damage by around twenty percent compared to the first four Iowas.

Her funding was authorized via the passage of the Two-Ocean Navy Act by the U.S. Congress on 19 July 1940, and she would now become the fifth Iowa-class ship built for the U.S.

[10] Funding for the battleship was provided in part by proceeds from the auction of "King Neptune", a Hereford swine presented across the state of Illinois as a fundraiser, ultimately helping to raise $19 million in war bonds.