The 3.7 cm Flak 18/36/37/43 was a series of anti-aircraft cannon produced by Nazi Germany that saw widespread service in the Second World War. The cannon was fully automatic and effective against aircraft flying at altitudes up to 4,200 m. The cannon was produced in both towed and self-propelled versions. Having a flexible doctrine, the Germans used their anti-aircraft pieces in ground support roles as well; 37 mm caliber guns were no exception to that. With Germany's defeat, production ceased, and, overall, 37 mm caliber anti-aircraft cannon fell into gradual disuse, being replaced by the Bofors 40 mm gun, and later, 35-mm anti-aircraft pieces produced by Switzerland.