In 1935 the Admiralty took a radical step by deciding that the next generation of aircraft carriers would be afforded the same protection as the big-gun units. Previous carriers had been armoured, but only only the lower or main deck over the machinery and magazines and in a waterline belt. The "Illustrious" Class ships were to have a hangar protected against 500lb bombs and 6" shells; this meant armouring the flight deck, and extending the vertical armour upwards to meet it. The flight deck between the lifts was 3" thick and the hangar walls, like the side belt, were 41" thick. As 5000 tons of armour had to be worked into a treaty limit of 23,000 tons, Illustrious was consequently considerably shorter than Ark Royal, and because the flight deck armour weighed 1500 tons, the second hangar deck was omitted to reduce the freeboard by 22 feet and preserve stability. The flight deck armour of the ships was penetrated only once - by an 1100lb (500kg) bomb which struck Illustrious during a concerted attack on her by German dive-bombers on 10 January 1941. That and six other bomb hits kept her out of action until the following December. Both Formidable and Victorious were struck by Kamikazes in 1945, but both were operating aircraft again soon after the hits - unlike the wooden-decked US carriers.
Throughout her career HMS Formidable operated mainly in the Mediterranean, where she, suffered bomb damage that kept her out of action. Her initial role was to accompany a convoy to Capetown from December 1940 to January 1941. Subsequently she was called to the Mediterranean to replace HMS Illustrious in February 1941.
She took part in the Battle of Cape Matapan 27-29 March 1941, and in the two following months was involved in convoy escort. She supported Crete operations in May where she suffered serious damage in air attacks by 1000kg bombs on 26 May 1941, and so was out of action for six months. Between June till December 1941 she underwent repairs in the USA, after which she sailed for the Indian Ocean remaining there between March-August 1942.
She returned in October 1942 to the Mediterranean and remained on station there until October 1943, where she took part in the North African landings in November 1942, Sicily landings in July 1943, and Salerno landings in September 1943. On completion of her Mediterranean tour of duties she took part in an Arctic convoy in October 1943.
HMS Formidable was refitted between January - June 1944 and then her aircraft were involved in the operation "Mascot" attack on the German Battleship Tirpitz in Norway on 17 July 1944. She took part in further attacks on Tirpitz 22, 24 and 29 August 1944 as part of the "Goodwood" operations.
HMS Formidable sailed for the Far East on 16 September 1944, being stationed at Gibraltar between September 1944 till January 1945 after machinery breakdown. She finally joined the British Pacific Fleet in place of HMS Illustrious on 16 April 1945. She subsequently took part in air strikes against Sakishima Gunto between April-May 1945. Between 4-9 May 1945 she was hit by Kamikazes - yet was able to operate aircraft within a few hours of attack, her aircraft later took part in air strikes against Japanese home islands between July-August 1945.
HMS Formidable arrived in Sydney, Australia on 23 August, subsequently undertook trooping voyages to UK September 1945-November 1946.
Formidable was reduced to reserve at Rosyth in March 1947, stricken 1950 and sold in 1953.
She was scrapped at Inverkeithing from November 1956.