Schwerer Gustav Railway Gun
The most powerful field gun (31.5 inch caliber) that has been deployed in the World War 2 theatre.
Was in service from 1941-1945 and saw action mainly in the Eastern Front: most notably, at the Battle of Sevastopol and the Siege of Leningrad.
As a World War 2 fan, I was fascinated by the mind blowing size and stats of this field gun (could launch a 7 tonne shell to a range of 29 miles) and it was only a matter of time before I attempted to replicate this monster as a 3D model.