 In addition, the extended range made possible by the use of drop tanks enabled the Merlin-powered Mustang to be introduced as a bomber escort with a combat radius of using two 2-piece, sheet-metal stamped construction drop tanks.The range would be further increased with the introduction of an self-sealing fuel tank aft of the pilot's seat, starting with P-51B-5-NA ("block 5").When this tank was full, the center of gravity of the Mustang was moved dangerously close to the aft limit.