The Panther was in development in the middle of the War, as a heavy tank to hold the German lines. Much of it's current design is modelled after the highly effective Soviet T-34 tank. It is a large tank, dangerous with a 75 mm cannon and 2 machine guns. It does, however have a reputation for mechanical problems.
Panther on Wikipedia

KwK 42 cannon:

Initially the Panther used a short 75 mm round and the tank was used largely in an infantry support role, but after the gun was switched out with the KwK 42, which launched long 75 mm rounds, the Panther became the main battle tank of the Germans. However the diameter of it's cannon was smaller then the Tiger's 88 mm cannon, it even supass it in some parameters, on shorter distances. The combination of the power of the Panther, and the ability of the German tank commanders, at one point even enabled 11 Panthers to destroy over 60 Soviet T-34s.
Calibre - 75 mm (3.46 inch)
Maximum Velocity - 3030 ft/s (925 m/s)