Defense and Freedom has a very interesting post about the Tank und Flieger (TuF, tank and aircraft) machine gun. The first prototypes were introduced just before the end of WWI …

One of the few German development programs against this new problem was the development of the granddaddy of all heavy calibre machine guns: The Tank und Flieger (TuF, tank and aircraft) machine gun. It was basically an enlarged Maxim machine gun with a more powerful cartridge that offered the necessary penetration power to turn tanks into swiss cheese and it had the necessary external ballistic performance to serve as air defence in a good radius.