The design of the BMW 801's cowling was key to its proper cooling, which BMW designed and built themselves and supplied with the engine. The design evolved throughout the war, including an extension to the engine mounts that allowed for larger cooling gills. This factory-supplied cowling also improved the simplicity of engine replacement in the field in more completely "unitizing" a BMW 801 radial engine, with as many of its auxiliary systems as possible being simultaneously replaceable with the engine itself, as opposed to opening or removing a "separate" cowling attached to the fuselage of the aircraft.