The XF4U had something like 600 or 800 changes before going into production, The Main visual differences are slightly longer nose and cockpit being moved back to fit in the fuselage fuel tank. After that changes to the F4U's airframe were minor. Changing from six .50 cal guns to four 20mm cannon may have required a slew of engineering drawings but the wing span, cord, airfoil, size of the flaps and ailerons all stayed the same.
Likewise on the P-36/P-40.
By the time you even get to the P-40 the Hawk 75 airframe was on it's 6th or 7th engine/power plant. It had gone from 3760lbs empty to 5,417lbs empty for the XP-40 and from 4843lbs loaded to 6,260lbs normal gross weight.
So are all the Hawk 75 versions separate aircraft or different versions/models of the same aircraft?
Wing is unchanged in size and shape, tail is pretty much unchanged, fuselage from the firewall back (except for the XP-37 and YP-37s) is pretty much unchanged.
Airframe may be beefed up to handle the higher weights.
Export Hawk 75s got four gun wings (2 to a side) well before the P-40B did so the gun and ammo layout and access hatches/doors predate P-40B.
XP-40 is the 10th production P-36A airframe, serial 38-10.
The 4th P-36A was turned into the XP-42
View attachment 588313
from the firewall back it was a standard P-36A.
SO when does the P-40 become a significantly different plane than the Hawk-75/P-36?
With the XP-40?
With the P-40 though P-40C?
With the P-40 D & E?
With the late P40F and the P-40Ks with the extended fuselage?