Prior to WWII, Ferdinand Porsche had designed and built air-cooled diesel engines for bomber aircraft. He tried to convince Hitler and others that they were totally practical, less complicated, and would be cheaper to build, run, and maintain than aircraft using water-cooled petrol engines.
Unfortunately for him, (and probably fortunately for us), the German High command were too impressed by the high-powered water-cooled engines of the Schneider Trophy winners, and thought that was the only way to go for aero engines.