Pak 43 and KwK 43
At the time Rheinmetall developed Flak 41, Krupp's tried to compete with their 8.8 cm Gerät 42 proposal, but it was not accepted for production as an anti-aircraft gun. Krupp continued development, resulting in the dreaded 8.8 cm Pak 43 anti-tank gun and 8.8 cm KwK 43 tank gun.
Pak 43 (abbreviation of Panzerabwehrkanone 43) used a new cruciform mount with the gun much closer to the ground, making it far easier to hide and harder to hit. It was also provided with a much stronger and more angled armour shield to provide better protection. All versions were able to penetrate about 200 mm of armour at 1,000 m, allowing it to defeat the armor of any contemporary tank. The standard armament of the Tiger II, the KwK 43 tank gun, was essentially the Pak 43 externally modified to fit into a turret. There were also self-propelled versions of the gun, including the lightly-armored Nashorn and, later, strongly-armored Jagdpanther tank destroyers.
Read more about this topic: 8.8 Cm Flak 18/36/37/41