The four ships in this class came about when the contracted supplier of the main armament for the GreekbattleshipSalamis being built in Germany was unable to supply due to the British blockade. The company — Bethlehem Steel in the United States — instead offered to sell the four 14 in (356 mm) twin gun turrets to the Royal Navy on 3 November 1914. The Royal Navy was using obsolete pre-dreadnought battleships for shore bombardment in support of the army in Belgium, and a design for a shallow-draught warship (known as "Monitors") suitable for shore-bombardment was quickly designed and built to use these turrets. The ships were laid down and launched within six months.
The ships carried a single main gun turret forward of a tripod mast, which was itself in front of a single funnel. A secondary armament of two 12-pounder (76 mm) guns was fitted, with a single 3-pounder (47 mm) anti-aircraft gun and a 2-pounder pom-pom completed the ships armament.
The monitors had a box-like hull, with very bluff bow and stern, and were fitted with anti-torpedo bulges. In order to speed construction, it was intended to use off-the shelf merchant ship engines, giving about 2,000 indicated horsepower (1,500 kW), which were expected to drive the ships to 10 knots (19 km/h; 12 mph). The rushed design, however, meant that the ships were much slower than expected — Raglan 's engines gave 2,310 indicated horsepower (1,720 kW) but the ship could only reach 61⁄2 knots (12.0 km/h; 7.5 mph).
During the planning and build, they were to be the Styx-class named after four American figures; GeneralUlysses S. Grant, General Robert E. Lee, AdmiralDavid Farragut and General Stonewall Jackson. Because the United States was still a neutral power at that time, using these names would have been undiplomatic and so they were simply called M1 through M4 before receiving their final names.
The design included a seaplane for spotting the guns, but it was found that land-based aircraft were more effective; as monitors, they would never operate in the open sea, and storing the seaplane on top of the turret meant it had to be removed to avoid damage, even if not required before the guns could fire.