I am trying to design in mission builder a scenario and I can't get 4 B-17G's to climb above 7500m with 100% fuel and 16 500lb bomb payload. The ceiling on the plane is suppose to be 10800m. Can someone explain why the altitude is limiting me to 7500m?

A plane's official ceiling doesn't necessarily mean what its capable of with a full load of bombs. Probably...the B-17 will not be able to climb any higher until the bombs or some of the fuel is gone. The AI will do its best to try. I haven't tested that in a while but you're probably reaching the B-17s limit.

My great uncle used to fly Halifax's during WWII and he said the plane always rose up like a feather in the wind after dropping the payload. Until then it was extremely heavy.

The RAF tried B-17s on very high altitude missions.

Their ditty went something like this.

"We are flying Flying Fortresses at 30,000 Feet
We are flying Flying Fortresses at 30,000 Feet
We`ve bags of point five ammo and a teeny weeny bomb
and we drop the b****rd from so high we don`t know where its gone"