That would be the "extended range fuel pod" although they did also have a "cargo pod" but I haven't seen any photos of this mysterious option.
The pod attached over the bomb bay and was droppable in the event of combat, or the need to drop ordnance although the pod was used mostly on the A-20G gunships and for ferrying.
Here's an A-20G with a pod in place - click on the photo for the large version for better details.