A lot of people who are new to the SP community want to build bomb bays but can't.
So this is the solution, first step is to make the plane itself. (make it out of fuselage) Now after you made the shape replace some of the fuselage with hollow fuselage. Then make it 50% bottom fill. After this make another piece of hollow fuselage, make this one top fill 50% and left fill 50%, put this on the right side of the fuselage. After that you need to mirror the right side, then fill with whatever bombs you like. The last step is to attach the bay doors to small hinges, then you are finished.
My plane right here that I made using these steps here
Thanks for the help I have been trying to figure out how to make a bomb bay for weeks now but when you ask the internet it comes up with very little help and a lot of that "help" is so hard to understand that they're completely confused me by the first step.