Reports coming in from the air war in Europe lead to the Air Combat Command and the Air Materiel Command issuing an order that all combat aircraft currently under construction be fitted with certain equipment that would make them more "combat capable". The USAAF also specified that all aircraft with these capabilities be give a "D" suffix.
The result of these change was that the remainder of the initial order for 66 P-38s were completed as P-38D-LO. The model designation remained 222-62-02. Serials were 40-774/809.
The P-38D differed from the P-38 in having a low-pressure oxygen system, self-sealing fuel tanks, a retractable landing light, and provision for flares. A change in tailplane incidence, together with a redistribution of elevator mass balances, increased the mechanical advantage of the elevator control, resulting in the elimination of buffeting and facilitating dive recovery. The P-38D featured a new low-pressure oxygen system, which supplanted the old high-pressure oxygen system of earlier versions. This system became standard on all subsequent production models. Normal fuel capacity remained 210 gallons, but maximum internal fuel was reduced from 390 to 340 gallons.
The P-38D had a maximum speed of 390 mph at 25,000 feet. An altitude of 20,000 feet could be reached in 8 minutes. Service ceiling was 39,000 feet. The first P-38Ds began to reach USAAF units in August 1941.
For a brief period, the USAAF considered naming the P-38 "Atlanta". However, the P-38D and subsequent versions were officially christened "Lightning" by the USAAF.
Changes were taking place at such a rapid rate that even the changes introduced on the P-38D did not really make it combat-ready. In 1942, the P-38 was redesignated RP-38 and the P-38D was redesignated RP-38D, the 'R' prefix meaning 'restricted to non-combat roles'. They were used strictly as combat trainers.
P-38s, RP-38s, and some P-38Ds from Selfridge Field were active participants in the September 1941 Army-Navy joint maneuvers in Louisiana, but few of the actually had any guns installed. By the time of Pearl Harbor, there were only 69 P-38 and P-38D fighters on strength.