Through all the modifications leading from XP-38 to P-38H, the basic contours of the engine nacelles of the Lightning had remained virtually unchanged. The P-38J version, which first began to appear in August of 1943, introduced some appreciable differences in the geometry of the engine nacelles which make this and later versions easily distinguishable from earlier versions of the Lightning.

    Earlier P-38s had passed the compressed air from the turbosuperchargers through a hollow passageway lying along the leading edge of the wing all the way from boom to wing tip and back in order to cool it down before it entered the carburetor. There were problems encountered with this arrangement. The difficulty in controlling the superchargers caused frequent engine backfires, some of which actually caused changes in the shape of the wing leading edge. The large area of these wing intercoolers also make them vulnerable to gunfire. The P-38J (Model 422) introduced a revised powerplant installation, with the intercooler being changed to a core-type radiator located below the engine. The air intake for the intercooler was sandwiched between the oil radiator intakes in a deeper, lower nose. The core-type radiator took cooling air through the central duct behind the propeller and exhausted it through a controllable exit flap, thus permitting a considerable amount of control over the the temperature of the air entering the carburetor. The leading edge tunnels were eliminated and were replaced by additional self-sealing fuel cells in the outer wing panels. The modification was initially tested on P-38E [41-1983].

    P-38J also had redesigned Prestone coolant scoops on the tail booms. All P-38Js retained the V-1719-89/-91 engines of the P-38Hs, but their more efficient cooling installations enabled military rating at 27,000' to be increased from 1240 to 1425hp, and war-emergency rating was 1600hp at that altitude.

    The revised beard radiators produced some additional drag, but it was more than adequately compensated for by the improved cooling which made the Allison finally capable of delivering its full rated power at altitude. Consequently, the P-38J was the fastest variant of the entire Lightning series—420 mph at 26,500'. Maximum speed at 5,000' was 369 mph, 390 mph at 15,000'. Range was 475 miles at 339 mph at 25,000', 800 miles at 285 mph at 10,000', and 1,175 miles at 195 mph at 10,000'. Maximum range was 2260 miles at 186 mph at 10,000 feet with two drop-tanks. An altitude of 5,000' could be attained in 2 minutes, 15,000 feet in 5 minutes, 10,000' in 7 minutes. Service ceiling was 44,000'. Weights were 12,780# empty, 17,500# normal loaded, 21,600# maximum. Armament consisted of one 20mm Hispano M2(C) cannon with 150 rounds plus four .50 Colt-Browning MG 53-2 machine guns with 500 rounds per gun. In addition two 500#, 1000# or 1600# bombs or ten five-inch rockets could be carried on underwing racks.

The 1,010 Model 422-81-14s included three production blocks. The first block consisted of 10 service test P-38J-1s. These were quickly followed by 210 P-83J-5s with two 55-US gallon additional fuel tanks in the leading edge space previously occupied by the intercoolers and thus restoring maximum internal fuel capacity to 410 gallons (1010 gallons with drop-tanks). Modifications, including the addition of stiffeners, were required to prevent deformation of the new wet wing leading edge. The last production block consisted of 790 P-38J-10s with flat windshields with the bulletproof glass panel being incorporated into the windshield.

    These were followed by Model 422-81-22s in two blocks. The first block consisted of 1400 P-38J-15s with revised electrical systems. The second block consisted of 350 P-38J-20s with modified turbo regulators.

    When earlier J-series Lightnings went into a high speed dive, their controls would suddenly lock up when a certain speed was reached and the nose would begin to duck under, making recovery from the dive very difficult. The problem began at Mach 0.65 to 0.68, accompanied by vigorous buffeting and a strong nose-down pitch. As speed increased, it became progressively more and more difficult to recover from the dive, larger and larger stick forces being required. At Mach 0.72 dive recovery became for all practical purposes impossible, and runaway dives that far out of hand usually had fatal results. The onset of severe buffeting would, of course, provide adequate warning for a pilot in a diving P-38 that he was about to have a problem, but it was easy to get distracted during the stress of combat. The problem was so severe that the Lightnings found it very difficult to follow enemy fighters in a dive and many escaped unscathed.

    The problem was eventually traced to a shock wave that formed over the wings as the Lightning entered the transonic regime, the shock wave preventing the elevators from operating. In order to counteract this problem, starting with the P-38J-25 (Model 422-81-23) production block, a small electrically-operated dive flap was added undereach wing, outboard of the engine nacelles and hinged to the main spar. The dive flaps would change the characteristics of the airflow over the wing, offsetting the formation of the shock wave and permitting the elevators to operate properly. That innovation largely solved the problems encountered by diving P-38s.

The P-38J-25 production block also introduced power-boosted ailerons operated by a hydraulically-actuated bellcrank and push-pull rod, making it easier for a pilot to maneuver the airplane at high airspeeds. The boosting system was one of the first applications of powered controls to any fighter, and required only 17% of the previous stick forces. The system vastly improved the roll rate and thereby increased effectiveness in combat. P-38Js with power-boosted ailerons had the highest roll-rates of any fighter.

    In March 1944 Col Benjamin Kelsey reached an indicated speed of more than 750 mph during a high-speed dive in a P-38, which would have made the P-38 the first supersonic fighter. However, it was later discovered that compressibility effects on the airspeed indicator at about 550 mph had given a greatly exaggerated reading. Nevertheless, the Lightning handled well at high speeds, and its strong airframe withstood the excessive aerodynamic loading produced by the dives.

    With the increased use of the Lightning as a light bomber, the type was modified to carry in place of the forward-firing armament either a bombardier with a Norden bombsight in a glazed nose, or a "Mickey" BTO (Bombing Through Overcast) radar in the nose with an operator station between the radar and the pilot's cockpit. Thse modifications were developed at the Lockheed Modification Center in Dallas. The so-called "droop-snoot" Lightnings were used to lead formations of P-38s, each carrying two 2000# bombs which were released on instructions from the lead bombardier.

Two P-38J-20s [44-23544, 23549] were modified in Australia in autumn of 1944 for use as single-seat night fighters with AN/APS-4 radar in a pod under the starboard wing. The modifications were tested in New Guinea and the Philippines.

    P-38J-5 [42-67104] was tested at Wright Field and Orlando as a two-place night fighter with a radar operator sitting on a jump seat just aft of the pilot. The AN/APS-4 radar was initially mounted under the fuselage in a pod just aft of the nosewheel. The pod was easily damaged by stones thrown up by the nosewheel during take-offs and landings and was repositioned under the starboard wing, but that then resulted in interference from the adjacent engine nacelle.

    Beginning in Sep 1944 a P-38J was used to test an unique method for extending the range of escort fighters by having the fighter engage a hook trailed from a B-24H. Attached to the hook was a standard drop-tank. After contact, the tank was automatically attached to standard external tank fittings beneath the fighter's wing. The method proved to be basically feasible, but required considerable skill on the part of the Lightning pilot for it to work. Consequently, this innovation was not pursued any further.

    A number of P-38Js were modified in service as TP-38J two-seat "piggyback" trainers with a jump seat aft of the pilot. Some of these aircraft carried an AN/APS-4 radar pod underneath the starboard wing and were used to train P-38M crews. P-38J-1 [42-13565] was fitted with an experimental retractable ski installation.

    The few surviving P-38J aircraft were redesignated F-38Js in 1948 when USAAF became USAF and the P designation changed to F.

    POP: 10 P-38J-1 [42-12867/12869, 13560/13566 ]
    POP: P-38J-5 [42-67102/67311]
    POP: P-38J-10 [42-67402/68191]
    POP: P-38J-15 [42-103979/104428, 43-28248/29047, 44-23059/23208]
    POP: P-38J-20 [44-23209/23558]
    POP: 210 P-38J-25 [44-23559/23768]

In June 1944 the AAF supplemented Lockheed's production capacity with a order from Consolidated-Vultee at Nashville for 2,000 P-38L-5-VN fighters similar to the Lockheed-built P-38L-5-LO. Delays in getting a production line going resulted in only 113 P-38L-5-VNs being delivered by the end of the war. Shortly after V-J Day the remaining 1,887 of the order were cancelled—a similar fate befell 1,380 P-38L-5-LOs then on order from Lockheed.

    When the war ended, large numbers of P-38Ls were scrapped or sold as surplus. The small number still remaining in USAF service in 1948 were redesignated F-38L.

    POP: 1,290 P-38L-1-LO [44-23769/25058]
    POP: 2,520 P-38L-5-LO [44-25059/27258, 44-53008/53327]; cancelled: [44-53328/54707]
    POP: 113 P-38L-5-VN [43-50226/50338]; cancelled: [43-50339/52225]