Northrop Grumman Corporation produced 21 B-2 Spirit aircraft from 1993 to 1997 in Production Blocks 10, 20, and 30. Originally designed as a land-based all-weather intercontinental heavy bomber for the US Air Force to deliver nuclear weapons and penetrate sophisticated air defenses through low-observable stealth characteristics during the Cold War, it was later modified to deliver conventional and guided weapons as well. The B-2A model had a maximum speed of 680 mph, a service ceiling of 50,000 feet above sea level, and a range of 6,000 miles that was unlimited with aerial refueling. It could carry upwards of 50,000 lbs. of conventional, guided, and nuclear weapons and was operated by a crew of two pilots. The aircraft was 69 feet long, had a wingspan of 172 feet, stood 17 feet high, and had a maximum takeoff weight of 375,000 lbs. It was powered by four General Electric F118-GE-100 turbofan engines providing 19,000 lbs. of thrust each. The B-2 was the largest stealth aircraft and the largest flying wing aircraft, was considered the most survivable aircraft in the world, and at $2 billion each was the most expensive aircraft. Its low observability came not only from a reduced radar picture but also from a unique combination of reduced infrared, acoustic, electromagnetic, and visual pictures.