 The most unusual feature of the aircraft was its use of " Metalite " for its skin . This was made of balsa that was sandwiched between two thin sheets of aluminum . " <unk> " was also used for the surfaces of the vertical stabilizer and rudder ; this was similar to Metalite , but used fiberglass instead of aluminum . Two fuel tanks were fitted in the center of the fuselage . The forward tank , ahead of the wing , contained 220 US gallons ( 830 l ; 180 imp gal ) and the rear tank , 150 US gallons ( 570 l ; 120 imp gal ) . These were supplemented by two jettisonable 140 @-@ US @-@ gallon ( 530 l ; 120 imp gal ) tip tanks . The cockpit was well forward and was provided with a bubble canopy which gave the pilot good visibility . He was provided with a Mk 6 lead @-@ computing gyro gunsight . Underneath the cockpit were four 20 mm ( 0 @.@ 79 in ) M3 autocannon . Their 600 rounds of ammunition were carried behind the pilot . The empty casings of the two upper guns were retained in the aircraft , while those from the two lower guns were ejected overboard . 
