There is a tutorial at YouTube that is like 12 parts (more or less) on making a jetliner (in either Max or Gmax). One of those parts of the tutorial is making that odd belly assembly. Search for 'tutorial, jet, jetliner, making, Max, Gmax' and I am sure you will find it at YouTube.
I recently made this on the Learjet. Its not as pronounced as this one, but has one. I forget how I did mine.
If you are making half of the fuselage (one side, then clone/mirror) that takes out part of the complications. Second, you might just take your time and study how to do it. What helps me is to make a drawing of the shape with cross section lines (grid) on a piece of paper and that helps me to 'see' in my mind how a mesh could be done to accomplish a shape.
One way I would do is is make the middle area of it with several polygons, like a belt, starting at its top side location where it begins, and loop that down under. Next, start extruding the Edges outwards to the front, then to the rear. Then start pinching the front (and later the rear) and get them to blend into the fuselage. Then cut the fuselage away where the belly will be, Attach it, then start welding Vertices together so its nice and smooth.
You might have to do it a few times to get it right. Just remember to backup your work, save up as you go so you can import/merge a new fuselage or belly part again if you need to start over, etc.