HMS Indomitable was a modified Illustrious-class aircraft carrier built for the Royal Navy during World War II. Originally planned to be the fourth of the class, she was redesigned to enable her to operate more aircraft, 48 instead of 36. A second hangar was added above the original, raising the flight deck by 14 feet (4.3 m), although the hangar-side armour had to be reduced to compensate. The lower hangar was made shorter than the upper hangar due to the need for extra workshops and accommodation to support the added aircraft.

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.