The four ships in this class came about when the contracted supplier of the main armament for the Greekbattleship 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 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 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 guns was fitted, with a single 3-pounder 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, which were expected to drive the ships to. The rushed design, however, meant that the ships were much slower than expected — Raglans engines gave but the ship could only reach. During the planning and build, they were to be the Styx-class named after four American figures; General Ulysses S. Grant, General Robert E. Lee, AdmiralDavid Farragut and General Stonewall Jackson and they were launched under these names. Because the United States was still a neutral power at that time, using these names would have been undiplomatic and so they were renamed as simply M1 through M4 before completion, then 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.
