[Graham Boak]

With bombers, there was always a trade-off between payload and range, as few could carry the combination of maximum fuel and maximum weapons. So the amount of fuel carried would depend upon the target, and hence the weapon load.

I would expect fighters and fighter-bombers to be full at (almost) all times, because of the need for combat and/or loiter. An overload case for a fighter-bomber might be different - I don't know for sure that a Typhoon could take-off with its full fuel load when loaded with two 1000lb bombs, for example. One other possibility might be for the use of the 8th AF long range fighters over Normandy: the P-51 was required to empty its fuselage tank before taking part in manoeuvres, so for these (comparatively) short range operations they may not have used this tank.