Aircraft fuel tanks, systems and methods increase an aircraft's fuel capacity. The fuel tanks have a tank body defining an interior space for holding aircraft fuel, and a relief manifold assembly operatively associated to the tank body to prevent an overpressure condition within the interior space of the fuel tank body. The relief manifold assembly preferably includes a buffer vessel defining a buffer chamber in fluid communication with the interior space defined by the fuel tank body. The buffer vessel may advantageously be fixed to the tank body within the interior space thereof. At least one of a fuel vent manifold assembly for venting the interior space of the fuel tank and a fuel refill/transfer manifold assembly for supplying fuel to and withdrawing fuel from the interior space of the fuel tank. At least one control box (e.g., containing valves, pumps and/or sensors) external of the fuel tank may be provided so as to fluid-connect the at least one fluid manifold assembly to the main fuel system of the aircraft. A plurality of aircraft fuel tanks may therefore be positioned adjacent to one another, preferably within the fuselage (e.g., a cargo compartment) of the aircraft so as to be disposed generally along a longitudinal axis of the aircraft.