HMS Duncan was the lead ship of the six-ship Duncan class of Royal Navy pre-dreadnought battleships. Built to counter a group of fast Russian battleships, Duncan and her sister ships were capable of steaming at 19 knots (35 km/h; 22 mph), making them the fastest battleships in the world. The Duncan-class battleships were armed with a main battery of four 12-inch (305 mm) guns and they were broadly similar to the London-class battleships, though of a slightly reduced displacement and thinner armour layout. As such, they reflected a development of the lighter second-class ships of the Canopus-class battleship. Duncan was built between her keel laying in July 1899 and her completion in October 1903.