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Almost as much discussion (and dissension!) on this subject as the Chicken/Egg debate, but for what it's worth, and hopefully to shed a little more light on this nagging question that has been perplexing Mustang modelers for decades, here is what I was able to find. I am NOT a Mustang authority, and I am not qualified to express an opinion, learned or otherwise, on this subject, so please consider that before launching any flak in my direction...as they used to say, "Don't shoot me, I'm just the piano player!" According to an NAA document I read on the SIG, the basic wing structure, airfoil, etc. was unchanged from the NA73X to the P-51D/K. The P-51F/G/H wings were structurally different. The only other significant changes noted in the numerous discussion topics concerning the wings of Mustangs leading up to the D and the D were the LE extension, main gear fairing doors, gun fit, and stiffeners ahead of the ailerons. The LE extension on all variants prior to the D had a downward camber- this also affected the shape of the wingroot fairing at the LE. On the D, the LE extension began further outboard and had no camber. The inner fairing doors on the D were not the same outline and shape compared to the earlier variants, mainly due to the retraction jacks and uplocks being moved from the front to the rear of the wheel bays. In addition, the stiffeners and shape of the bulkhead at the front of the wheelbay/LE extension was different on the D as compared to the earlier variants. The P-51B/C/D also had two chordwise stiffeners in front of each aileron on the upper surface and one on the lower surface of each wing. The reasons for the LE extension on the D was a subject of much discussion, from pilots, crew chiefs, and enthusiasts, as well as excerpts from NAA correspondence and documents, and the general concensus seemed to be the change could have been due to various reasons, but there was no unanimous winner. Possible reasons for the LE extension change given were: (1) simplification of the structure/parts count in that section of the wing (2) change due to the new fairing doors/retraction jacks (3) cg consideration- much discussion of the rearward shift in cg which began when the Merlin was fitted to the P-51B/C, quoting larger radiator/oil/cooler/intercooler/85gallon fuel tank; the LE extension increased fwd wing area and helped to bring the cg forward. It was also discussed that the D wing was not thicker than the wing on the previous variants because of the extra two guns fitted, or that this was done so that the .50cal guns could be mounted upright, versus being canted, as they were in the earlier variants that had .50cal wing guns. NAA evidently engineered the wing from the beginning to take heavier armament at some point, thus the reason to cant the guns to allow feed chutes for .50cal or 20mm guns to fit within the wing without blisters or fairings, so the guns were canted from the outset. Since it was determined that 20mm armament was not going to be fitted, the guns were mounted upright- this also alleviated to a great extent, the jamming that was taking place on the B/C models. I will try to find at which point on the wing the LE extensions began, probably will be at which wing station this took place, not inches/feet from the wingroot; If I can find this, I will add it to this post. Hope this helps; a lot of people much smarter than I am are still undecided on this topic, but there is no disputing the fact that the LE extensions, main gear fairing doors and retraction jacks, and the forward section of the wheel bays were different between pre-D's and D's; I guess for those of us who model, it's not important knowing why these changes were made, but how the changes affected the appearance. I have four of the old Monogram 1/72 P-51B's, and if nobody can see their way clear to release an accurate wing, I'm gonna start cutting mine up after I figure out which kit/s would be the best recipient... pretty amazing that Monogram got this often overlooked part right so many years ago! Whew- I'm pooped! Mike