16 4.5" AA guns (8x2)
48 2pdr AA (6x8)
33 aircraft (later increased)

This class were the first aircraft carriers with fully armoured hangars to be built for any navy, and an examination of the armouring shows that resistance to 6in shells or to a destroyer's guns at close range was at least as important as that to bombs. The weight of the armour permitted only one hangar, and the flight deck was 38ft above the deep load waterline.

There was the usual island and funnel, and the flight deck had an effective length of 620ft in Illustrious and 670 ft in the other two, later increased to 740ft in all by reducing the round downs. The width remained 95ft, and there was a single catapult capable of launching 14.000lb at 66kn. The hangar measured 458ft x 62ft x 16ft and the two rectangular lifts (which could take 14.000lb) 45ft x 22ft. Petrol stowage was 50.540gal, which was insufficient for the 52-54 aircraft it was found possible to carry in 1944-45 using a permanent deck park with outrigger stowage. The belt amidships was taken to the hangar deck, to the flight deck only on the starboard side abreast the island, and to the upper deck for the last 30ft fore and aft. The hangar side armour was complete and extended well beyond the belt ends, merging with the belt only by the island, and the flight deck was 3in over the hangar armour and 1½in at the ends and below the island. The lifts were not armoured. The hangar deck was 1in outside the citadel where it formed the hangar floor and 3in-2½in between the hangar walls and the belt, whilst the lower deck was 3in aft. Underwater protection was similar to Ark Royal's but improved in detail, and the designed GM at deep load was 8.26ft. Tactical diameter was about 1050yds., and there were 3 boiler and three engine rooms. The 4.5in guns were in low crowned 'between decks' mountings.

Modifications;
The light AA armament was altered during the war Illustrious eventually having 40 2pdr (5x8), 3 40mm (3x1) and 52 20mm Oerlikons. Formidable had 482pdr (6x8), 12 40mm (12x1) and 34 20mm Oerlikons while Victorious had 40 2pdr (5x8), 21 40mm (2x4, 2x2, 9x1) and 45 20mm Oerlikons.