HMS Formidable was an Illustrious-class aircraft carrier ordered for the Royal Navy before the Second World War. After being completed in late 1940, she was briefly assigned to the Home Fleet before being transferred to the Mediterranean Fleet as a replacement for her crippled sister ship Illustrious. Formidable's aircraft played a key role in the Battle of Cape Matapan in early 1941, and they subsequently provided cover for Allied ships and attacked Axis forces until their carrier was badly damaged by German dive bombers in May.

The Illustrious class was a class of aircraft carrier of the Royal Navy that included some of the most important British warships in the Second World War. They were laid down in the late 1930s as part of the rearmament of British forces in response to the emerging threats of Nazi Germany, Fascist Italy and Imperial Japan.