Based in Letchworth, Hertfordshire in England, SD was started in 1922 by Harry Shelvoke and James Drewry, both of whom had successful careers in commercial vehicle design and manufacture with Lacre Lorries. During the Second World War, like most British manufacturing companies, S&D's entire output was devoted to the war effort, producing aircraft parts, equipment for landing craft and tanks, and even a miniature submarine (the Welfreighter) at their Letchworth plant. The W-type of the 1950s originally had a SD 3.9 litre petrol (gasoline) engine but latter had a Perkins P4 series diesel.