The V-3 was a German World War II large-caliber gun working on the multi-charge principle whereby secondary propellant charges are fired to add velocity to a projectile. Two full-size guns were built in the underground Fortress of Mimoyecques in northern France and permanently aimed at London, but they were rendered unusable by Allied bombing raids before completion. Two smaller guns were used to bombard Luxembourg from December 1944 to February 1945.