[Graham Boak]

Not quite. Strengthening would be needed fore-and-aft for recoil forces, but the main strength of a wing is required to carry the lift forces and these point upwards. Adding weights out along the wing permits a lighter spar structure by relieving this upward load. This is the principle behind pylon-mounted engines, although here they can also act as anti-flutter devices.

And to flog a tired horse, the Mustang wing was not truly laminar. Nor was its wing-mounted armament particularly heavy. The outstanding features of the Mustang were the integration of the radiator into the fuselage using expansion/contraction ducts (leaving space for significantly larger fuel capacity) and the very high standard of external finish achieved.