The Royal Navy's Formidable class of battleships were an eight-ship class of predreadnoughts designed by Sir William White and built in the late 1890s. The class is often further divided into a separate London class, and the London class sometimes is divided further into a separate Queen class.
The Formidables were similar in appearance to and had the same armament as the Majestic and Canopus classes that preceded them. The Formidables are often described as improved Majestics, but in design they really were enlarged Canopuses; while the Canopus class took advantage of the greater strength of the Krupp armor employed in their construction to allow the ships to remain the same size as the Majestics with increased tonnage devoted higher speed and less to armor without sacrificing protection, in the Formidables Krupp armor was used to improve protection without reducing the size of the ships. The Formidables thus were larger than the two preceding classes, and enjoyed both greater protection than the Majestics and the higher speed of the Canopus class. The Formidables' armor scheme was similar to that of the Canopuses, although, unlike in the Canopuses, the armor belt ran all the way to the stern; it was 215 feet (65.5 m) long and 15 feet (4.8 m) deep and 9 inches (229 mm) thick, tapering at the stem to 3 inches (76.2 mm) thick and 12 feet (3.7 m) deep and at the stern to 1.5 inches (38.1 mm) thick and 8 feet (2.4 m) deep. The main battery turrets had Krupp armor, 10 inches (254 mm) on their sides and 8 inches (203 mm) on their backs.
The Formidables improved on the main and secondary armament of previous classes, being upgunned from 35-caliber to 40-caliber 12-inch (305-mm) guns and from 40-caliber to 45-caliber 6-inch (152-mm) guns. The 12-inch guns could be loaded at any bearing and elevation, and beneath the turrets the ships had a split hoist with a working chamber beneath the guns that reduced the chance of a cordite fire spreading from the turret to the shell and powder handling rooms and to the magazines.
The Formidables had an improved hull form that made them handier at high speeds than the Majestics. They also had inward-turning screws, which allowed reduced fuel consumption and slightly higher speeds than in preious classes but at the expense of less maneuverability at low speeds.
After the first three Formidables, there was a change in design for the last five ships, starting with ; as a result they are often considered to constitute the London class, but also can be viewed as in effect a sub-class of the Formidable class. The main difference in the Londons was thinner deck armor and some other detail changes to the armor scheme. and the consequent lower displacement.
The last two London class ships to be built, and , were identical to the other Londons except that they had open 12-pounder gun batteries mounted in the open on the upper deck amidships and had a lower displacement. Queen and Prince of Wales were laid down after the Duncan class battleships that succeeded the Formidables and Londons, and were completed after the Duncans as well. They generally are considered part of the Formidable or London class, but the difference in the mounting of their 12-pounder guns, their lower displacement, and their later construction than the Duncans lead some authors to viewed them as constituting a Queen class separate from the Formidable and London classes.
The last of the ships to commission, Prince of Wales, was the last battleship for which Director of Naval Construction Sir William Henry White had sole design responsibility. She also was the last of the 29 battleships of the Majestic, Canopus, Formidable, London, Duncan, and Queen classes, commissioned between 1895 and 1904, which had all been based on the single, standard Majestic design and reached their final development in Queen and Prince of Wales.
Like all predreadnoughts, the Formidables, Londons, and Queens were outclassed by the dreadnought battleships that began to appear in 1906. However, they continued in front-line duties through the early part of World War I.
The ships saw peacetime service in the Mediterranean, Atlantic, and home waters. With the appearance of the new dreadnought -type battleships and battlecruisers beginning in 1906, predreadnoughts such as the Formidables, Londons, and Queens were consigned to less demanding roles for much of the First World War, during which two were lost in action and a third was destroyed by an accidental explosion. Early war service in home waters was followed by duty in the Mediterranean including the Dardanelles campaign. The survivors were discarded soon after the war ended.