The original concept was based upon an outline design similar to that for HMS Dreadnought, but with improved armament.
The ship was conceptually constructed from three components, several outline studies being produced by Nathaniel Barnaby.

A heavily armoured citadel 75 feet (23 m) wide and 110 feet (34 m) long was located amidships, which would keep the ship afloat and stable regardless of what happened to the ends.
This citadel contained the main guns, the boilers and the engines.

The ends were unarmoured, but with a 3-inch-thick (76 mm) armoured deck 6–8 ft below the waterline to limit damage to the underwater section to keep them buoyant.
Coal bunkers were located over the armoured deck and surrounded by 4-foot-wide (1.2 m) compartments filled with cork.
The ship had bunker capacity for 400 tons of coal below the deck for use during combat, when the above-deck bunkers would be inaccessible and possibly flooded.
The structure above the armoured deck also contained a large number of watertight compartments to further preserve buoyancy.
There was also light superstructure to provide crew accommodation, and freeboard in rough weather, although anticipated to be seriously damaged in any major engagement.

Barnaby wanted a ship both broader than existing designs to mimimise rolling and as short as possible to reduce its size as a target.
Making a ship broader compared to its length was known to reduce its speed, so the innovative technique of water tank tests on models, pioneered by William Froude, was used to finalise a design.
This was ten feet wider than Duillo and twenty-one feet shorter, the smallest ever ratio of length to breadth in a metal first class warship.

Once the outline design was agreed, the detailed architectural design was done by William White and she was laid down at Portsmouth Dockyard on 24 February 1874.
The original concept was based upon an outline design similar to that for HMS Dreadnought, but with improved armament. The ship was conceptually constructed from three components, several outline studies being produced by Nathaniel Barnaby.
A heavily armoured citadel 75 feet (23 m) wide and 110 feet (34 m) long was located amidships, which would keep the ship afloat and stable regardless of what happened to the ends. This citadel contained the main guns, the boilers and the engines.
The ends were unarmoured, but with a 3-inch-thick (76 mm) armoured deck 6–8 ft below the waterline to limit damage to the underwater section to keep them buoyant. Coal bunkers were located over the armoured deck and surrounded by 4-foot-wide (1.2 m) compartments filled with cork. The ship had bunker capacity for 400 tons of coal below the deck for use during combat, when the above-deck bunkers would be inaccessible and possibly flooded. The structure above the armoured deck also contained a large number of watertight compartments to further preserve buoyancy. There was also light superstructure to provide crew accommodation, and freeboard in rough weather, although anticipated to be seriously damaged in any major engagement.
Barnaby wanted a ship both broader than existing designs to mimimise rolling and as short as possible to reduce its size as a target. Making a ship broader compared to its length was known to reduce its speed, so the innovative technique of water tank tests on models, pioneered by William Froude, was used to finalise a design. This was ten feet wider than Duillo and twenty-one feet shorter, the smallest ever ratio of length to breadth in a metal first class warship.
Once the outline design was agreed, the detailed architectural design was done by William White and she was laid down at Portsmouth Dockyard on 24 February 1874.
HMS Inflexible (1876)