The Flakpanzer IV "Wirbelwind" (Whirlwind in English) was a German self-propelled anti-aircraft gun based on the Panzer IV tank. It was developed in 1944 as a successor to the earlier Mobelwagen self-propelled anti-aircraft gun. In the first years of World War II, the German military forces had less interest in developing self-propelled anti-aircraft guns, but as the Allies began to gain air superiority, the need for more mobile and better-armed self-propelled anti-aircraft guns increased.
The Panzer IV's turret was removed and replaced with an open-top, nine-sided turret that housed a 2 cm Flakvierling 38, a quadruple mount of 20 mm cannon. A closed-top design would have been preferable, but this was not possible due to the heavy smoke generated by the four anti-aircraft guns. The shape of the turret earned it the nickname Keksdose ("biscuit tin"). The combination of armor and rapid fire from the four guns of the Wirbelwind made it very effective against lightly armoured ground targets such as trucks and armored cars; infantry were particularly vulnerable. Between 87 and 105 Wirbelwinds were converted from repaired Panzer IV chassis, but due to discrepancies between the recorded production numbers, the actual number may never be known.