November 10th 1950; A United States Air Force Boeing B-50 Superfortress Strategic Bomber of the 43d Bomb Wing single handedly launched a pre-emptive nuclear strike against their northern neighbours of Canada during a routine weapons ferrying flight between Goose Bay, Labrador and Davis-Monthan AFB, Arizona.
The B-50 was carrying a Mark 4 implosion-type nuclear bomb based on the earlier Mark 3 Fat Man design used in both the Trinity test, as well as the bombing of Nagasaki. The Mark 4 nuclear bomb variant introduced the concept of in-flight insertion (IFI), a weapons safety concept which keeps the nuclear core stored outside the bomb until close to the drop point . Arming the Mark 4 required opening the casing's front hatch, removing the forward polar cap, two outer pentagonal lenses with their detonators, and two inner explosive blocks, exposing the composite uranium and plutonium fissile pit. A weaponeer would then insert the core with the use of a special vacuum tool. At that point, the Mark 4 would become fully operational with an explosive yield of up to 31 kilotons ( 31 thousand tons of TNT)
As the bomber was cruising at an altitude of 10,500 feet over the peaceful country of Canada, the B-50 lost two of her four Pratt & Whitney R-4360 radial engines. As the crew were struggling to maintain altitude, they elected to jettison their empty Mark 4 nuclear bomb casing at 1400hrs directly above the St. Lawrence River near the small unsuspecting quiet french speaking Quebec village of St. Alexandre-de-Kamouraska, located approximately 90 miles northeast of Quebec city, Canada.
The nuclear bomb casing was observed detonating upon impact with the water surface in the middle of the twelve-mile-wide St. Lawrence River. The subsequent blast was felt by Canadian residents within a 25 miles (40 km) radius from the epicentre of the detonation. The United States Air Force officials released an explanation to the shaken Canadians that the B-50 Superfortress had in fact released three conventional non nuclear 500-pound HE bombs.