In my opinion it was the Paris gun. Built by the Germans during WW1, effective firing range of 81 miles, it's projectiles are thought to be the first man made objects sent into space(their firing arc). read more
This was the Paris Gun, one of the largest ground artillery pieces ever made and fielded. Lemme give you a quick perspective on how big this was. Yes, that is a German soldier in the bore of the Paris Gun. read more
The artillery system features a fully automated 155mm/L52 gun and a Remotely Controlled Weapon Station (RCWS) based on a 6×6 Volvo commercial articulated vehicle. The main gun delivers an intensive rate of fire of 21 rounds per 3.5 minutes and can fire all types of 155mm projectiles including advanced sensor-fused and precision-guided munitions. read more