Republic Aviation's P-47 Thunderbolt, also known as the "Jug" (because either it was shaped like a squatted milk jug, or it was short for "juggernaut"), was the largest, heaviest, and most expensivefighter aircraft in history to be powered by a single piston engine. It was heavily armed with eight .50-caliber machine guns, four per wing. When fully loaded, the P-47 weighed up to eight tons, and in the fighter-bomber ground attack roles could carry five inch rockets or a significant bomb load of 2,500 pounds; over half the weight the B-17 bomber could carry on long-range missions. The P-47, based on the powerful Pratt & Whitney R-2800 Double Wasp engine, was to be very effective as a short-to-medium range escort fighter in high-altitude air-to-air combat and when unleashed as a fighter-bomber, proved especially adept at ground attack in both the World War II European and Pacific Theaters.