The P-47 Thunderbolt, also known as the Jug, was the largest single-engined fighter of its day, and a vast improvement over its predecessor. It was one of the main United States Army Air Force (USAAF) fighters of World War II. The Thunderbolt also served with a number of other Allied air forces. The P-47 was effective in air combat but proved especially adept in the ground-attack role. It had eight 50 caliber machine guns (four per wing). When fully loaded the P-47 could weigh up to eight tons. Its modern-day equivalent in that role, the A-10 Thunderbolt II, takes its name from the P-47.