HMS B11 was the last of 11 B-class submarines built for the Royal Navy in the first decade of the 20th century. Completed in 1906, it is best known for carrying out a successful attack on the Ottoman battleship Mesûdiye in the Dardanelles, an action for which her captain received the Victoria Cross. It spent the remainder of its active life serving in the Mediterranean, being converted into a surface patrol boat late in the war. B11 was sold for scrap in 1919.

The B-class was a class of 11 submarines, built by Vickers in Barrow-in-Furness for the Royal Navy, and launched in 1904–06. One boat was sunk by a collision in 1912, but the remainder served in World War I. Three boats protected the transfer of the British Expeditionary Force to France in 1914, but were soon relegated to local defence and training duties. Six submarines were in the Mediterranean when the war began and were quickly sent to the Dardanelles to prevent a breakout by the German battlecruiser SMS Goeben and the light cruiser SMS Breslau into the Eastern Mediterranean. B11 ventured into the Dardanelles in December 1914 and sank the elderly Turkish ironclad Mesudiye.