Enough with the history lesson, onto the what-if
Suppose that a properly combat capable Lightning – the P-38D or even better the P-38E – was in service, in squadron strength, in the summer of 1940.
What you have is a 375-395 mph capable single seat fighter with the capability to get to 20,000 ft in 7.5 to 8 minutes.
It has very long loiter time – around 4 hours – and good range of 750-800 miles.
It has a heavy armament of 4 .50 cals and 1 x 20 mm, although the US made Hispano was not particularly reliable at this point of the war. It is also a very good gun platform, being stable and benefiting from its contra-rotating props. Guns would also jam during heavy maneuvering.
It is easy to fly for an experienced pilot, but is a highly complex fighter that is difficult to learn and difficult to master for an inexperienced pilot. The cockpit arrangements were less than ideal – even the British though they were sub-standard – and the cockpit heating was poor and the canopy had a tendency to fog badly.
Its a maintenance hog. With two engines and two turbos, it was the most maintenance intensive single-seat fighter in the US inventory.
Its not really suitable for combat above 25,000 ft. The early P-38s had problems with carburettors, intercoolers and turbo-chargers above 25,000 ft. There were also some problems with lateral instability above 20,000 ft. The aircraft was also limited in its dive speeds due to compressibility.
Even though it's a tricycle landing gear aircraft, it was not particularly suited to landing on the soft and bumpy Fighter Command emergency grass strips.
What this adds up to for me is this: The P-38 offered definite combat advantages over the Spitfire and Hurricane – better speed, heavier armament, longer range and loiter time.
However, these are not enough to offset its strategic disadvantages. As a complex and heavy twin engine fighter, it required much more learning time and skill from a novice pilot that a single-engine fighter.
The RAF's chief problem was getting pilots trained and into the air. Having an aircraft that required more training than a single-engine fighter (the USAAF recommended a minimum of 30 hours in S/E fighters and then a number of transition flights in lower performance twin-engine aircraft before transitioning to the P-38) means that the RAF will be able to get less fighters into the air.
The aircraft was maintenance heavy, taking significantly more man-hours than a single seat fighter to service and to get back into the air.
The aircraft still had teething problems that would have proved particularly aggravating for the RAF in the Battle of Britain. Those intercooler and turbo issues to a long time to sort and aren't the sort of thing you want to be dealing with while your airfields are getting bombed and strafed.
With its poor soft field performance and high maintenance requirements, it would have to be deployed well back from the coast, at airfield with concrete runways. So it would have been less useful than the Spitfire or Hurricane as a pure interceptor, despite its roughly comparable or even slightly better time to climb performance.
The aircraft's problems at altitude would have also made it much less suitable to the final stages of the Battle of Britain, much of which was fought at very high altitudes. The Hurricane, even the Mk II, really struggled in this phase as well.
If you could accelerate the development of the P-38 by three or so years, putting it into service with the RAF in mid 1939, then I can see it being a success. With a sorted high speed aircraft with 2 or 3 times the firepower of a Spitfire or Hurricane, the P-38 would have been a terror to German bomber crews.
Three or four squadrons, with experienced pilots and good ground crews, could have been a very useful for certain roles.
With its long loiter times, the aircraft would have been excellent for medium altitude combat patrols along the English coast. You could put up 4 ship or 8 ship formations and have them run intercepts while the Spitfires and Hurricanes climb and gather.
The P-38 also could have been very useful for long-range intercepts in the North. If a raid is detected out to sea, the could have scrambled and gotten to the bombers off the coast, possibly blunting a raid before it had ever gone feet dry.