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 and Mediterranean Seas.
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 that much weight so high in the ship allowed only a single-storey hangar due to stability concerns.
This halved the aircraft capacity of the Illustrious class compared with the older unarmoured carriers, trading offensive potential for defensive survivability.

Formidable had a length of 740 feet (225.6 m) 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 men upon completion in 1940.
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 30.5 knots (56.5 km/h; 35.1 mph).
On sea trials, Formidable reached 30.6 knots (56.7 km/h; 35.2 mph) with 112,018 shp (83,532 kW).
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).

The 753-foot (229.5 m) armoured flight deck had a usable length of 670 feet (204.2 m), due to prominent "round-downs" at bow and stern to reduce air turbulence, 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 ship could accommodate up to 54 aircraft rather than the intended 36 after the adoption of "outriggers" on the flight deck during the war and the flattening of the "round-downs" that increased the usable length of the flight deck to 740 feet (225.6 m) to facilitate the use of a permanent deck park.
The additional crewmen, maintenance personnel and facilities needed to support the extra aircraft severely crowded the ship.
She was provided with 50,650 imperial gallons (230,300 l; 60,830 US gal) of aviation gasoline.
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 and Mediterranean Seas. 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 that much weight so high in the ship allowed only a single-storey hangar due to stability concerns. This halved the aircraft capacity of the Illustrious class compared with the older unarmoured carriers, trading offensive potential for defensive survivability.
Formidable had a length of 740 feet (225.6 m) 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 men upon completion in 1940. 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 30.5 knots (56.5 km/h; 35.1 mph). On sea trials, Formidable reached 30.6 knots (56.7 km/h; 35.2 mph) with 112,018 shp (83,532 kW). 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).
The 753-foot (229.5 m) armoured flight deck had a usable length of 670 feet (204.2 m), due to prominent "round-downs" at bow and stern to reduce air turbulence, 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 ship could accommodate up to 54 aircraft rather than the intended 36 after the adoption of "outriggers" on the flight deck during the war and the flattening of the "round-downs" that increased the usable length of the flight deck to 740 feet (225.6 m) to facilitate the use of a permanent deck park. The additional crewmen, maintenance personnel and facilities needed to support the extra aircraft severely crowded the ship. She was provided with 50,650 imperial gallons (230,300 l; 60,830 US gal) of aviation gasoline.
HMS Formidable (67)