the hatch at the top is so that if a bomb set fails to release someone can come arm and drop them manually... of course the problem is the box to do so is below the bombs XD

you can see the box just above the yellow bar on the right. the bar is just to stand on

the bombs are 100lb and this is the entire bomb bay, a single pair of 2 bomb racks with about 6-7 stacked vertically