Unlike which happened with the contemporary American and Japanese aircraft carriers, projected to operate in the Pacific where the threat from land-based aircraft was not very important, the British aircraft carriers of the Illustrious class were destined to the narrow waters of the North Sea and the Mediterranean, where the threats coming from land were more important and the risk of surface attacks was considerable. Because of this their hangar was protected against 152-millimeter projectiles and 250-kilogram piercing bombs dropped from 2000 meters of altitude.

The limitations imposed by the Washington Treaty had restricted the displacement of British warships to 23370 tonnes. To be able to accommodate the extra weight in a just slightly increased displacement, the Illustrious was built with only one hangar deck instead of the two that the Ark Royal had, which reduced to a half the number of aircraft that she could carry (36 in total). Albeit the radar would render the aircraft carriers as less vulnerable to air attacks, the decision of armoring the hangar was justified in that time given the normalized protection and the valid displacement limit.

The Illustrious had anti-aircraft cannons which in their totality equalled the artillery power installed in the Ark Royal. However the 114 millimeters turrets were installed in a higher position which allowed limited firing arcs across the flight deck, but this procedure was not very used because of the risk of hitting the own aircraft or crew members. The anti-aircraft defense was excellent thanks to four fire control directors for the 114 millimeters cannons. The secondary armament consisted of six 40 millimeters octuple mountings of the type known as "pom-pom".

The propulsion was given by three propellers like in the Ark Royal, but the machinery was somewhat more powerful. The Illustrious was authorized the 13th January 1937 and her construction started the 27th April of that same year. She was launched the 5th April 1939 and when she was completed, the 21st May 1940, Britain was already immerse in a conflict of large proportions.

The Illustrious started her career in the Mediterranean, taking part in the air attacks against Benghazi (16-17 September 1940) and Taranto (11-12 November 1940), and escorting convoys in route to Malta (in September-October 1940 and from November 1940), until resulting seriously damaged by six bombs the 10th January 1941. Field reparations were carried in Malta on 10-23 January, under a constant air attack, and then she was transferred to United States, where she arrived in May 1941. The reparations lasted until November 1941 in the shipyards at Norfolk and she returned to Britain in December of the same year.

In March-April 1942 she was transferred to the Indian, where she took part in the landings at Madagascar in May 1942, serving there until January 1943, when she returned to Britain for being upgraded with new installations. After a brief service in the Norwegian coasts in July 1943, she returned to the Mediterranean, where she took part in the landings at Salerno in September 1943. She returned to United States in October 1943, and from that moment her destination would be the Far East. Transferred to the Indian in January 1944, she took part in the attacks against the Dutch East Indies (April-August 1944) and Palembang (December 1944 - January 1945). Later in the Pacific, she would be damaged by a kamikaze attack in April 1945, during the invasion of Okinawa.

The Illustrious survived the war to be used for trials and training until December 1954, when she was put in the reserve, to be finally scrapped in 1956.