They were based on the British Power Boat Company Type Two 63 ft HSL (high-speed launch) originally designed for the Royal Air Force for air-sea rescue but reduced to in length. They could carry two torpedoes and achieve a maximum speed of . The Royal Navy ordered their first (of a total of 18) in 1936. These entered service as MTB numbers 1 to 12 and 14 to 19. In the early days of the war, they were painted with different numbers and photos distributed to the press to give the impression the Royal Navy had more than they actually did. One photo was sent to the American monthly "Popular Science" showing the number twenty-three.