P-38J suffered from exploding turbo chargers at altitudes of 30,000, where the B-17s typically flew, and therefore, coupled with their limited range, were unacceptable as long range bomber escort aircraft.
The P-51 could hold its own in a dogfight but only after it dropped its external fuel tanks. Maneuverability was hampered by these fuel tanks (especially if they were loaded with fuel), to a point where the P-51 was easy prey to the enemy’s aircraft guns.
A B-17 carried 4 tons of bombs and 1.5 tons of machine gun ammo. The US 8th Air Force shot down 6,098 fighter planes, that's one fighter plane for every 12,700 shots fired.