Continuous improvements to existing fighters occurred from the beginning of the war. Initially slowly and later at an ever faster pace.

Usually, the new model appeared in small quantities in selected units and after a time, the older models were sent to schools or secondary theaters.
This infographic shows the introduction of newer models at three different times: During the Battle of Britain (1940), during the Non-stop Offensive (1941), and at the time of the Dieppe raid (1942).

At the peak of the German offensive in 1940, the Luftwaffe equipped all its Gruppen with the Bf109E-1, E-3, and E-4. They were almost identical in performance except for the E-1 which lacked cannons. The Spitfire Mk II had just appeared on the front and only one squadron was available.

One year later, the bulk of the fighters in FC were Spitfire IIs and Hurricane IIs, which faced the 109 E-7. Six Spitfire squadrons boasted the brand new Mk V while the newer 109F-1/F-2 equipped a third of JG 26's Gruppen.
Another year later and the Mk V was now the most numerous British fighter, although a small number of squadrons had the new Mk IX. By that time the excellent Fw 190 A-1, A-2, and A-3 (that outperformed the Spitfire Mk V) equipped all fighter Gruppen on the Western Front.

We will publish a comparison of the fighters in service in 1941 in the next week's infographic.
.