Compared to iron, steel allowed for greater structural strength for a lower weight. France was the first country to manufacture steel in large quantities, using the Siemens process. At that time, steel plates still had some defects, and the outer bottom plating of the ship was made of wrought iron.
All-steel warships were later built by the Royal Navy, with the dispatch vessels Iris and Mercury, laid down in 1875-1876.
The Italian Navy had started constructing a pair of battleships, Duilio and Dandolo, equipped with four Armstrong 15-inch (381 mm) guns weighing 35 tons each. These were superior to the armament of any ship in the British Mediterranean Squadron, and Inflexible was designed as a counter to them.
Packed with innovations, Inflexible mounted larger guns than those of any previous British warship and had the thickest armour ever to be fitted to a Royal Navy ship. Controversially, she was designed so that if her un-armoured ends should be seriously damaged in action and become water-logged, the buoyancy of the armoured centre section of the ship would keep her afloat and upright.
The ship was the first major warship to depend in part for the protection of her buoyancy by a horizontal armoured deck below the water-line rather than armoured sides along the waterline.