Don't forget that the B-17 could carry an overload (including external racks under the wings) of 17 600lb. Don't think it was used much, if ever, operationally due to parasite drag & other performance issues, but it proved that the basic design could haul a significant bombload.
Remember that the B17 was originally designed to threaten the US Navy's role of protecting the US mainland from an enemy navy. Therefore range and endurance were more important than absolute bombload.