You may have something there.
You can actually find some accounts or even manuals that will list not only the 12,800lb internal bomb load but a max bomb load of 17,600lbs.
A pair of 4000lb bombs (American not British cookie) under the wings and six 1600lb AP bombs inside.
And again it is a total useless load except for bombing an entire city, assuming the B17 when so loaded could even make it past Dunkirk and make it back to England without running out of fuel.
The American 4000lb bomb was short and fat and wasn't going to land anywhere near where the 1600lb AP bombs were going to land.
And unless the target was the already motioned thick steel or a number of feet of concrete a normal 500lb bomb held more explosive.
A B-17 on normal structures would do more damage with eight 500LB bombs than with eight 1600lb bombs.
View attachment 666783
4000lb bomb on the right.
flying the B-17 with a pair of those metal parachutes hanging off the wings didn't do anything for the speed and range either.
However in 1940-41 there wasn't a big need to exaggerate the B-17s load because British 4 engine bombers were just trickling into service (like the B-17) and the British twin engine heavies couldn't carry much either. A 1940 Wellington with Pegasus engines didn't carry what a 1943 Wellington with Hercules engines did.