The Bliss–Leavitt Mark 8 torpedo was the United States Navy's first 21-inch by 21 feet torpedo. Although introduced prior to World War I, most of its combat use was by PT boats in World War II. The torpedo was originally designed in 1911 by Frank McDowell Leavitt of the E. W. Bliss Company and entered full mass production in 1913 at the Naval Torpedo Station in Newport, Rhode Island. It was deployed on destroyers and battleships during World War I and cruisers built in the 1920s. All US battleships and most cruisers had their torpedo tubes removed by 1941. The Mark 8 remained in service through World War II on older destroyers, primarily the Wickes and Clemson classes. It also equipped PT boats early in World War II, but was replaced by the Mark 13 torpedo on most of these in mid-1943.

A PT boat was a motor torpedo boat used by the United States Navy in World War II. It was small, fast, and inexpensive to build, valued for its maneuverability and speed but hampered at the beginning of the war by ineffective torpedoes, limited armament, and comparatively fragile construction that limited some of the variants to coastal waters. In the US Navy they were organized in Motor Torpedo Boat Squadrons (MTBRONs).