Designed for service on the Amazon River, the ships were of shallow draft and heavy armament and were ideally suited to inshore, riverine and coastal work but unsuitable for service at sea, where their weight and light draft reduced their speed from a projected twelve knots to under four.

The three Humber-class monitors were originally ordered for the Brazilian Navy as the Javary-class gunboats intended for inshore work on the River Amazon and its tributaries.

Vickers attempted to find a foreign buyer for the boats and the British government stepped in to purchase the gunboats on 4 August 1914 for £155,000 each in order to prevent them being bought by a neutral navy and then sold on to Germany.

During the Battle of the Frontiers and subsequent operations in 1914, the Humber-class monitors were all employed in bombarding German batteries and positions, under the command of Rear-Admiral Horace Hood.

Only the long-range guns of the shallow-draft monitors could reach the hidden cruiser, and although the journey to East Africa took nearly six months under tow from Malta, the monitors were ultimately successful in destroying the German ship, their shells directed by two seaplane observers.