[Graham Boak]

Well, a lighter spar and hence lighter wing is an advantage not to be sneered at. I was merely pointing out that statements about strengthening wings was not as clear-cut as you suggested. As the original poster pointed out, by no means the majority of successful WW2 fighters had the masses concentrated such as you describe, practical disadvantages (such as fuselage size leading to excessive drag) being one counting against any gain in agility. Part of the trade-offs for all fighters.

As for armament, at the time the Mustang was designed the P-47 had eight 0.5 guns, the RAF standard for new fighters was 4x20mm cannon, the Zero and Fw 190 were flying. The first Mustang was the Mk.1, which had weaker armament (a mix of machine guns) than the majority of its contemporaries, largely cannon-armed. The following Mk.1A (P-51 no suffix) had 4x20mm, but this did not see mass production. The mass production P-51B/C had only four 0.5 machine guns, arguably the weakest of contemporary fighters other than the Ki 43 (although some of the Yaks could also be included, perhaps). As it was highly successful, heavy armament cannot have been a key factor.