I am really sorry to dredge a popular topic up again. Unfortunately, after reading through some papers on the P-51 as well as all the past forum threads I could find on this topic, I have found myself very confused about the source of the P-51(B or D)'s drag advantages over the Spitfire Mk IX.
First, let's establish some facts.
In flight tests with equivalent engines & power-plants, the P-51B/D was 25-30 MPH faster than the Spitfire in level flight. Drawing from David Lednicer's (a professional aerodynamicist's) comparison of WW2 fighters (Source: http://www.wwiiaircraftperformance.org/mustang/Lednicer_Fighter_Aerodynamics.pdf), he states "with the same version of the Rolls-Royce Merlin engine and propeller installed, the Mustang X was measured to be 23 mph faster than the Spitfire IX....The P-51B is even faster than the Spitfire."
The Mustang had lower total drag than the Spitfre. Again, showing a table from Lednicer's article, which uses drag estimates from flight tests and wind tunnels of the era. (Note: he notes in the article that he has been conservatively high with the P-51's drag).
So we know that the Mustang was faster than the Spitfire and had lower total drag.
However, I have been confused out of my mind as to what technical features account for this lower drag. Can this community come to the rescue?
Now, the most obvious answer to this question would be that the P-51B/D incorporated an airfoil section that was designed to maintain larger regions of laminar flow on it, lowering the skin-friction drag of the wing. However, to quote Lednicer again: "in service the aircraft was unlikely to have a substantial laminar flow on the wing and transition occurred in the first 15% of the chord".
So it's unlikely that significantly reduced skin friction drag on the wing accounted for the lower drag unless someone has information that shows that whatever <15% extent laminar flow on the P-51B/D wings was markedly higher than similar extent of laminar flow on the wings of the Spitfire IX or other fighters of the era.