A PT boat was a motor torpedo boat (hull classification symbol "PT", for "Patrol Torpedo"), a small, fast vessel used by the United States Navy in World War II to attack larger surface ships. The PT boat squadrons were nicknamed "the mosquito fleet".
Torpedo boats were first developed in the early 20th century as an inexpensive way to deliver torpedoes which could destroy ships as heavy as battleships without the massive weight necessary for large caliber guns. By WWII, the initial mission of the American PT boats was to battle destroyers, which themselves were a shortened name for "torpedo boat destroyers". Though many would question the military effectiveness of the boats in this role, their psychological impact in deterring Japanese attacks would be just as important. The Navy was short on larger ships as they were just starting the manufacturing of a massive naval fleet which would come later in the war, and wood construction made strategic materials such as steel available for other uses. Later in the war, the boats were much more effective as gunboats against targets their own size, such as armored barges that the Japanese used to shuttle troops and supplies between islands.
Among the famous PT boats was PT-109, commanded by future United States President John F. Kennedy, an Elco PT-103 class torpedo boat. Another was PT-41, a 77 foot Elco boat commanded by Lieutenant John D. Bulkeley, who rescued General Douglas MacArthur from certain capture by the Japanese in a daring escape from Corregidor Island, Philippines, and was awarded the Medal of Honor for his exploits. This story inspired both a book, They Were Expendable, and a movie of the same name. This story of the diminutive PT boats beating overwhelming odds went a long way to prop up sagging American morale in the dark days after the attack on Pearl Harbor.