The Republic P-47 Thunderbolt, also known as the Jug, was the largest single-engined fighter of its day, and a vast improvement over the Curtiss P-40 Warhawk, its predecessor. It was one of the main United States Army Air Force (USAAF) fighters of World War II, and also served with other Allied air forces. The P-47 was effective in air combat but proved especially adept at ground attack. It had eight .50-caliber machine guns, four per wing. When fully loaded the P-47 could weigh up to eight tons. A modern-day counterpart in that role, the A-10 Thunderbolt II, takes its name from the P-47.