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.
Illustrious is best remembered for her strike on the Italian Fleet at Taranto on the night of 11-12 November 1940. Torpedoes from her ancient Swordfish aircraft sank one battleship and forced two others to be beached.
Before she went to the Far East, Victorious operated mainly with the Home Fleet, taking part in the hunt for Bismarck, launching strikes against Tirpitz in a Norwegian fjord, and covering Russian convoys. From 1950 until 1958 she underwent a major reconstruction, but served only another 10 years before a major fire during a refit and cuts in the RN’s carrier force level led to her paying off.
Formidable operated mainly in the Mediterranean, where she, too, suffered bomb damage that kept her out of action for 6 months. She spent another 6-month period out of action at the beginning of 1944, undergoing a refit to prepare her for Pacific operations.
As originally envisaged, the class comprised 6 ships, but the fourth ship and the last pair differed sufficiently for them to be regarded as separate classes.
The increase in size of aircraft, particularly jets, made post-war use of the Illustrious class impractical due to a lack of hangar height. Victorious was the only carrier to serve past the early 1950's after being extensively reconstructed to operate jet aircraft.

* Illustrious = 670'