Just been doing a little research on USAAF fighter deployments and production.
P-38 production was pretty slow before the F model. There were only 68 P-38s on hand for the USAAF by the end of 1941, all P-38D or earlier. There were only 207 P-38s built in 1941, compared to ~1400 built in 1942.
The first major production model is the P-38E with around 210 produced. Even the USAAF didn't really consider it a 'combat ready' airplane, that was reserved for the heavier and more powerful P-38F. Between January and September 1942, there were just 525 P-38F airframes delivered. The first USAAF engagement of LuftWaffe aircraft didn't come until August, 1942, when P-38Fs of the 33rd fighter squadron engaged a FW 200 Condor off the coast of Iceland and shot it down. By the end of August 1942 there were just 179 P-38s deployed in England. The first operation sorties were flown from England on 28 August, 1942.
P-51/A-36A production and deployment in the USAAF really doesn't take off until 1943. The USAAF didn't order the first production batch of P-51s for itself until April, 1942, almost 2 years after the first British orders. The first squadrons to operate the A36 were the 27th and 86th, who didn't see combat until mid 1943. The first USAAF P-51 Mustang units saw their first combat in a similar time period.
British deployment of the Mustang was understandably a little earlier. First combat was recorded at the end of July, 1942 and the first victories in August. By the end of October 1942 there are 14 Mustang I squadrons operating out of the UK, mostly in the tactical reconnisance and low level fighter regiemes. The P-51 was significantly quicker (25-45 mph) than the Spitfire V at all alts below about 20,000 feet, but it was 5 minutes slower to get to 20,000 feet and less manouverable above this height. Typhoons and Mustangs would of had handled the bulk of the low level interception duties, the Tyffie being 15-20 mph faster still than the Mustang.
The comparison here is Spitfire and Typhoon production and deployment. By September 1942, 515 Typhoons had been handed over to the RAF, with some 13 squadrons operating the type and more converting. Similar production numbers to the P-38, but they didn't have to do the trans-Atlantic flight and had the luxury of using existing facilities. In August 1942 the RAF had 42 squadrons of Mk V Spitfires, 4 Mk IX and 2 Mk VI and 8 with Mk IIb/c, all in the UK. There were also several fighter bomber Hurricane squadrons (rapidly being phased out in favour of the Typhoon) and 2 Westland Whirlwind squadrons, used for long range escort. So British fighters are still going to make up the bulk of the fighter forces in the UK.
I'd say that until at least late 1942 or early 1943, when the P-47C and P-38G really start to roll off the production lines, the main fighters of the USAAF would be the P-40 and P-39. Both are capable fighters in their element, but probably not quite up to the level of the Spitfire V and 109F or Typhoon Ib and 190A4/5 seeing service at the same time. I think both would of struggled to intercept raids coming in at anything above 20,000 feet.
The premium fighters in the USAAF by August 1942 would be the P-38F, with the Mustang I and P-47C entering the fight in a few months time. P-40s and P-39s would account for the bulk of the fighter force until well into 1943, possibly all the way to the end of the year?