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 defense provided by an armored flight deck for survival.
In the Illustrious class, armor was carried at the flight deck level and formed an armored box-like hangar that was an integral part of the ship's structure. This armor scheme was designed to withstand 1,000-pound bombs. The flight deck had an armored thickness of 3 inches.