The largest gun ever built, by at least a factor of 3, is the V-3 cannon, also known as the London Gun. At least two of these guns were built by Nazi Germany and put in bunkers in chalk cliffs in Mimoyecques, France. It is a common misconception that the guns were never built; they were, although the whole complex, which was originally intended to hold five guns, was never completed due to Allied bombing.
The V-3 guns, code-named Hochdruckpumpe, "High Pressure Pump," or Tausendfüßler, meaning "millipede," used a series of T-junctions filled with charges timed to explode just as the shell was passing along each segment of the the 460 ft. (140 m) barrel. This allowed muzzle velocities of over 4,920 ft./s (1,500 m/s) for a 308.6 lbs. (140 kg) shell, giving a range of over 102.5 miles (165 km). The V-3s, built at a fixed angle of 30° into the cliff, were intended to hit London, hence their alternate name. As of 2007, the London Gun is by far the largest gun yet built.
Construction on the London Gun began in September 1943 by slave workers. Hitler ordered the guns built as a terror weapon to augment the V-1 and V-2 rockets, overruling the German military, which feared the location chosen was too susceptible to bombing. The chalk cliff holding the London Gun was 98 ft. (30 m) tall, and protected by a 16 ft. (5 m) thick concrete dome. The concrete was so thick that even the 11,905 lb. (5,400 kg) Tallboy penetrator bombs used by the Allies proved useless at first.
Informed of the early construction of the London Gun almost immediately by the French Resistance, the Allies began bombing November 1943, only two months after construction began. On 6 July 1944, three Tallboys, purely by chance, made it down the shafts used to contain the gun barrels, reaching the lower level of the complex and killing dozens of workers. Work on the London Gun halted at this point, but unaware of their success, the Allies continued attacking the bunker using radio-controlled kamikaze airplanes escorted by controller planes. Unfortunately this strategy ended up being deadly, and several Allied pilots, including the brother of a future president, Joseph P. Kennedy, Jr., were killed in attacks on the complex.
Other very large guns include the railway guns Schwerer Gustav and Dora, built by the Krupp company, which were the largest artillery pieces ever fielded by an army during wartime. Weighing in at 2,963,012.8 lbs. (1,344 tonnes), they could fire a 15,432 lb. (7 tonne) shell over a distance of 23 miles (37 km), and played an active role during WWII. The largest gun ever planned for construction was the "Big Babylon" supergun, commissioned by Saddam Hussein, which was to be about 500 ft. (156 metres) long, with a bore of over 3 ft. (1 m), capable of launching a projectile into orbit and thereby being able to hit anywhere on the globe. Plans to build this gun fell apart when its designer was assassinated.