A U.S. Air Force B-52 Stratofortress collided with a KC-135 Stratotanker during aerial refueling over the Mediterranean Sea, dropping three hydrogen bombs on land near Palomares, Spain, and a fourth one into the sea.

The Boeing B-52 Stratofortress is an American long-range subsonic jet-powered strategic bomber. The B-52 was designed and built by Boeing, which has continued to provide support and upgrades. It has been operated by the United States Air Force (USAF) since 1955 and was flown by NASA from 1959 to 2007. The bomber can carry up to 70,000 pounds (32,000 kg) of weapons and has a typical combat range of around 8,800 miles (14,200 km) without aerial refueling.

The Palomares incident occurred on 17 January 1966, when a United States Air Force B-52G bomber collided with a KC-135 tanker during mid-air refueling at 31,000 feet (9,450 m) over the Mediterranean Sea, near the Spanish village of Palomares in Almería province. The collision destroyed the tanker, killing all four crew members, and caused the bomber to break apart, resulting in the deaths of three of its seven crew members. The B-52G was participating in Operation Chrome Dome, a Cold War airborne alert mission involving continuous flights of nuclear-armed bombers.

The Boeing KC-135 Stratotanker is an American military aerial refueling tanker aircraft that was developed from the Boeing 367-80 prototype, alongside the Boeing 707 airliner. It has a narrower fuselage and is shorter than the 707. Boeing gave the aircraft the internal designation of Model 717. The KC-135 was the United States Air Force (USAF)'s first jet-powered refueling tanker and replaced the KC-97 Stratofreighter. The KC-135 was initially tasked with refueling strategic bombers, but it was used extensively in the Vietnam War and later conflicts such as Operation Desert Storm to extend the range and endurance of US tactical fighters and bombers.

Aerial refueling (en-us), or aerial refuelling (en-gb), also referred to as air refueling, in-flight refueling (IFR), air-to-air refueling (AAR), and tanking, is the process of transferring aviation fuel from one aircraft to another while both aircraft are in flight. The two main refueling systems are probe-and-drogue, which is simpler to adapt to existing aircraft and the flying boom, which offers faster fuel transfer, but requires a dedicated boom operator station.