The .55 Boys (13.9×99mmB in metric) is an anti-tank cartridge used by the United Kingdom in World War II. It was designed for use with the Boys Anti-Tank Rifle.

The Boys anti-tank rifle (officially Rifle, Anti-Tank, .55in, Boys, and sometimes incorrectly spelled “Boyes”), is a British anti-tank rifle used during the Second World War. It was often nicknamed the “elephant gun” by its users due to its size and large 0.55 in (14 mm) bore.

There were three main versions of the Boys: an early model (Mark I) which had a circular muzzle brake and T-shaped monopod, built primarily at BSA in England; a later model (Mk I*) built primarily at the John Inglis and Company in Toronto, Canada, that had a rectangular muzzle brake and a V shaped bipod; and a third model made for airborne forces with a 30-inch (762 mm) barrel and no muzzle brake. There were also different cartridges, with a later version offering better penetration.

Although adequate against light tanks and tankettes in the early part of the war, the Boys was ineffective against heavier armour and was phased out in favour of the PIAT hollow charge weapon mid-war.