During the Second World War, the safe retrieval of pilots and aircrew who had been shot down over, or who had had to ditch in, the seas around the British Isles required the use of fast craft to reach them. This was especially critical during the Battle of Britain, as the loss of trained pilots could hasten the loss of the battle. The Royal Air Force Marine Branch was tasked with this responsibility and its motto of "The sea shall not have them" emphasized this daunting mission. Rescuing downed aircrew required the coordination of RAF Coastal Command aircraft to locate them and relay the position to rescue launches operated by the Royal Navy and RAF Marine Branch. However, RN assets were frequently limited for this purpose as they were often needed elsewhere. The RAF Marine Branch needed dedicated high-speed craft for rescues. The initial solution was the British Power Boats 64-foot High Speed Launch (HSL).

British Power Boat Company was founded by Hubert Scott-Paine in 1927. Scott-Paine was the first factory manager of the Supermarine Aviation Company, eventually becoming the owner. Supermarine is best known as the builder of the Spitfire fighter plane, its carrier- based version Seafire and amphibious Walrus to name a very few. These airplanes were built long after Scott-Paine sold Supermarine in 1923. After World War I, Scott-Paine became interested in motor boat racing, having experience in adapting aero engines to marine use. Scott-Paine purchased the Hythe Shipyard in 1927, making it the base for British Power Boats (BPB). The firm and Scott-Paine had early successes in motor boat racing with Miss England I and Miss Britain III.

Scott-Paine favored using the hard-chine hull, which has a distinct, sharp angle where the boat's sides meet the bottom, unlike a rounded soft chine. The benefits include reducing drag, thereby increasing speed, and providing stability and allowing for sharp turns. Hard-chine hulls do have a tendency to cause pounding in choppy seas. Fred Cooper, chief designer at BPB, was able to develop hard-chine hull lines that reduced pounding while maintaining speed. This resulted in a 23-foot cabin cruiser and speed boat. This new hull form was later adapted for the initial BPB MTB design.

During the mid-1930s, Scott-Paine presented to the RAF a design for a 64-foot that met their requirements. BPD had already sold the RAF fast seaplane tenders and armored target boats using hard-chine hulls. The RAF purchased a number of these craft. By 1939, the 64-footers were found to have some practical operational issues. The boat’s flat deck proved to be too wet and the stern too high for getting wounded aircrew out of the water and on board. Also, the space for stretchers was restricted making it very difficult to tend to the wounded. So, the RAF began looking for a better alternative to the 64-footer.

BPB had by this time developed a “whaleback” 63-foot MTB which had already been purchased by Sweden and Norway. The “whaleback” profile had distinctive curve to its deck and a humped cabin. The wide beam at the break of the deck, made the boat steadier and drier forward, and the lower freeboard aft, made it easier to bring downed aircrew and the wounded on board. In addition, all spaces on the boat could be accessed without going on deck and there was space for to up to eight stretchers with ample room to work. Thus, the 63-foot “whalebacks” appeared to be a good alternative to the 64-footer. Also, BPB could supply these boats without stopping production.

The “whalebacks” were officially designated the Type Two High Speed Launch (HSL). They were powered by three Lion engines with a rated speed of 36 knots. The boats were initially armed with a pair of 0.303-inch Vickers or Lewis machine guns enclosed in Armstrong-Whitworth turrets. One turret was placed atop the pilot house and another further aft atop the cabin. Later on, a single 20mm Oerlikon was fitted aft on a strengthened rear deck. The RAF purchased 72 of the Type Two HSLs. After some time, the Type Two HSL was superseded by the 68-foot “Hants and Dorset” or Type Three HSL that were also built by BPB.

The 63-foot MTBs that had been purchased by Norway and Sweden were commandeered by the British government and converted to RN MGBs. The four Norwegian boats became MGB 42-45 and the two Swedish boats became MGB 40 and 41. MGB 44 and 45 eventually became Polish S2 and S3. All of the MGBs were powered by two Merlin engines.