including 26 Squadron, saw action covering the assault on the ground. By 1943–1944, British Mustangs were used extensively to seek out V-1 flying bomb sites. The last RAF Mustang Mk I and Mustang Mk II aircraft were struck off charge in 1945. Army Co-operation Command used the Mustang’s superior speed and long range to conduct low-altitude "Rhubarb" raids over continental Europe, sometimes penetrating German airspace. The V-1710 engine ran smoothly at 1,100 rpm, versus 1,600 for the Merlin, enabling long flights over water at altitude before approaching the enemy coastline. Over land, these flights followed a zig-zag course, turning every six minutes to foil enemy attempts at plotting an interception. During the first 18 months of Rhubarb raids, RAF Mustang Mk.Is and Mk.Ias destroyed or heavily damaged 200 locomotives, over 200 canal barges, and an unknown number of enemy aircraft parked on the ground, for a loss of eight Mustangs. At sea level, the Mustangs were able to outrun all enemy aircraft encountered. The RAF gained a significant performance enhancement at low altitude by removing or resetting the engine’s manifold pressure regulator to allow overboosting, raising output as high as 1,780 horsepower at 70 in Hg. In December 1942, Allison approved only 1,570 horsepower at 60 in Hg manifold pressure for the V-1710-39. The RAF also operated 308 P-51Bs and 636 P-51Cs, which were known in RAF service as Mustang Mk IIIs; the first units converted to the type in late 1943 and early 1944. Mustang Mk III units were operational until the end of World War II, though many units had already converted to the Mustang Mk IV (P-51D) and Mk IVa (P-51K) (828 in total, comprising 282 Mk IV and 600 Mk IVa). As all except the earliest aircraft were obtained under Lend-Lease, all Mustang aircraft still on RAF charge at the end of the war were either returned to the USAAF "on paper" or retained by the RAF for scrapping. The last RAF Mustangs were retired from service in 1947. U.S. operational service Prewar theory Prewar doctrine was based on the idea "the bomber will always get through". Despite RAF and Luftwaffe experience with daylight bombing, the USAAF still incorrectly believed in 1942 that tightly packed formations of bombers would have so much firepower that they could fend off fighters on their own. Fighter escort was a low priority, but when the concept was discussed in 1941,