HMS Illustrious was the lead ship of her class of aircraft carriers built for the Royal Navy before World War II. Her first assignment after completion and working up was with the Mediterranean Fleet, in which her aircraft's most notable achievement was sinking one Italian battleship and badly damaging two others during the Battle of Taranto in late 1940. In January 1941, during Operation Excess, Illustrious was hit multiple times by German and Italian dive bombers and was repaired in the United States. After sustaining damage on the voyage home in late 1941 by a collision with her sister ship Formidable, Illustrious was sent to the Indian Ocean in early 1942 to support the invasion of Vichy French Madagascar (Operation Ironclad).

After returning home in early 1943, the ship was given a lengthy refit and briefly assigned to the Home Fleet. She was transferred to Force H for the Battle of Salerno in mid-1943 and then rejoined the Eastern Fleet in the Indian Ocean at the beginning of 1944. Her aircraft attacked several targets in the Japanese-occupied Dutch East Indies over the following year before Illustrious was transferred to the newly formed British Pacific Fleet (BPF). The carrier participated in the early stages of the Battle of Okinawa until mechanical defects arising from accumulated battle damage became so severe she was ordered home early for repairs in May 1945.

The war ended while she was in the dockyard and the Admiralty decided to modify her for use as the Home Fleet's trials and training carrier. In this role she conducted the deck-landing trials for most of the British post-war naval aircraft in the early 1950s. She was occasionally used to ferry troops and aircraft to and from foreign deployments as well as participating in exercises. In 1951, she helped to transport troops to quell rioting in Cyprus after the collapse of the Anglo-Egyptian treaty of 1936. She was paid off in early 1955 and sold for scrap in late 1956.

The Royal Navy's 1936 Naval Programme authorised the construction of two aircraft carriers. Admiral Sir Reginald Henderson, Third Sea Lord and Controller of the Navy, was determined not to simply modify the previous unarmoured Ark Royal design. He believed that carriers could not be successfully defended by their own aircraft without some form of early-warning system. Lacking that, there was nothing to prevent land-based aircraft from attacking them, especially in confined waters like the North Sea and Mediterranean. This meant that the ship had to be capable of remaining in action after sustaining damage and that her fragile aircraft had to be protected entirely from damage. The only way to do this was to completely armour the hangar in which the aircraft would shelter, but putting so much weight high in the ship allowed only a single-storey hangar due to stability concerns. This halved the aircraft capacity compared with the older unarmoured carriers, exchanging offensive potential for defensive survivability.

Illustrious was 740 feet (225.6 m) in length overall and 710 feet (216.4 m) at the waterline. Her beam was 95 feet 9 inches (29.2 m) at the waterline and she had a draught of 28 feet 10 inches (8.8 m) at deep load. She displaced 23,000 long tons (23,369 t) at standard load as completed. Her complement was approximately 1,299 officers and enlisted men upon completion in 1940. By 1944, she was severely overcrowded with a total crew of 1,997. After post-war modifications to convert her into a trials carrier, her complement was reduced to 1,090 officers and enlisted men.

The ship had three Parsons geared steam turbines, each driving one shaft, using steam supplied by six Admiralty 3-drum boilers. The turbines were designed to produce a total of 111,000 shp (83,000 kW), enough to give a maximum speed of 30 knots (56 km/h; 35 mph) at deep load. On 24 May 1940 Illustrious ran her sea trials and her engines reached 113,700 shp (84,800 kW). Her exact speeds were not recorded as she had her paravanes streamed, but it was estimated that she could have made about 31 knots (57 km/h; 36 mph) under full power. She carried a maximum of 4,850 long tons (4,930 t) of fuel oil which gave her a range of 10,700 nautical miles (19,800 km; 12,300 mi) at 10 knots (19 km/h; 12 mph) or 10,400 nmi (19,300 km; 12,000 mi) at 16 knots (30 km/h; 18 mph) or 6,300 nmi (11,700 km; 7,200 mi) at 25 knots (46 km/h; 29 mph).

The 753-foot (229.5 m) armoured flight deck had a usable length of 620 feet (189.0 m), due to prominent "round-downs" at each end designed to reduce the effects of air turbulence caused by the carrier's structure on aircraft taking-off and landing, and a maximum width of 95 feet (29.0 m). A single hydraulic aircraft catapult was fitted on the forward part of the flight deck. The ship was equipped with two unarmoured lifts on the centreline, each of which measured 45 by 22 feet (13.7 by 6.7 m). The hangar was 456 feet (139.0 m) long and had a maximum width of 62 feet (18.9 m). It had a height of 16 feet (4.9 m) which allowed storage of Lend-Lease Vought F4U Corsair fighters once their wingtips were clipped. The hangar was designed to accommodate 36 aircraft, for which 50,650 imperial gallons (230,300 L; 60,830 US gal) of aviation spirit was provided.

The main armament of the Illustrious class consisted of sixteen quick-firing (QF) 4.5-inch (110 mm) dual-purpose guns in eight twin-gun turrets, four in sponsons on each side of the hull. The roofs of the gun turrets protruded above the level of the flight deck to allow them to fire across the deck at high elevations. Her light antiaircraft defences included six octuple mounts for QF 2-pounder ("pom-pom") antiaircraft guns, two each fore and aft of her island, and two in sponsons on the port side of the hull.