The Illustrious class was a class of aircraft carrier of the Royal Navy that included some of the most important British warships in World War II. The 1st ship in this class was HM Illustrious. She was laid down on 27th April 1937, launched on 5th April 1939 and commissioned on 25th May 1940. The Illustrious class relied on their anti-aircraft armament and the passive defence provided by an armoured flight deck for survival.
In the Illustrious class, armour was carried at the flight deck level and formed an armoured box-like hangar that was an integral part of the ship's structure. This armour scheme was designed to withstand 1,000 pound bombs. The flight deck had an armoured thickness of 3 inches.