I think you'll find if you read Wikipedia "Fighter Command" that during 1941/42, the RAF was losing 4 fighters for every German one shot down. Below 15000 feet, the P-40C (Tomahawk IIB) was superior to both the Spitfire Vb and the Bf 109F-1/2, and the later models superior to the Bf 109F-4, so both ideally suited to the campaign in the Western Desert. As far as I know the Spitfire Vc was superior to the Bf 109F-4. I'm talking about the RAF's campaign over France in 1941, when most of the Spitfires available until the Summer were Spitfire II's with less than 100 Spitfire V conversions from earlier marks. The production Spitfire Vb's didn't arrive until the Summer, at which point the Bf 109F-4 had arrived. The first production Spitfire Vc wasn't made until October 1941 by which time the campaign over France was over.
It always went back and forth. For a while one side had the advantage then it shifted the other way.
The Bf 109F of any subtype was definitely superior to a Spit II and maybe a Spit Va. By the time you had substantial numbers of Spit Vb and Vc in North Africa (Summer of 42), they were definitely
not suffering 4-1 losses against Bf 109s, and there weren't enough Fw 190s to make a difference.
P-40s had some advantages below 15,000 feet and for a short period in 1941 they were doing really well against the early Italian fighters and to some extent, once they got there, the Germans too. But the Germans reacted quickly - they started to retire their older Bf 109E and brought in the F, and particularly JG 27 quickly adopted their tactics to the performance ceiling limitation of the P-40. They routinely climbed above them to attack from above. Thus for most P-40 pilots combat started with an attack from above and behind - something a skilled pilot could survive and adapt to but was by no means a comfortable situation. Only once continued combat caused the Bf 109s to lose sufficient 'E' were the P-40s able to fight back. They also apparently used overboosting their engines as high as 70" Hg which could confer ~1700 HP at some altitudes.
The fact that the Commonwealth was not using finger 4 / pairs of wingmen until late 1942 further exacerbated matters, as did the general lack of training for pilots especially some of the Colonial pilots like the South Africans. As a result, a lot of P-40s got shot down. In the first half of 1942 the Commonwealth was taking heavy casualties of P-40 and Hurricanes.
It came down to certain pilots, like the Australian Ace Clive Caldwell, to reform training standards such as with his shadow shooting, or Nicky Barr who was one of the first to realize that you could turn with P-40s (and Hurricanes) with high G turns. Operational level tactics also shifted in the last quarter of 1942 more toward going after the Germans over their own bases - I think initiated by the Americans.
Once the P-40F & L showed up, with a performance ceiling up to 20,000, the advantage of the Bf 109F series and early G series was diminished, partly because they didn't have time to climb so high up especially when attacked over their own fields (often the Germans would put 2 or 4 Bf 109s on CAP over their fields but that was not enough) or over tactical battlefields when the main target was a medium bomber flying at 12,000 feet. The Americans were also using 'Finger Four' tactics with two pairs of wingmen which helped considerably.
You also had P-38s in late 1942 and Spit IXs show up in early 1943. The latter in particular clearly meant doom for the Germans, it obviously dominated all the Bf 109 types and was able to handle the Fw 190 as well.
S
