and are best done with splines or nurbs.
Not in Blender, if you ask me. Unless you are comfortable shaping a surface by moving only its control-points around in space. But that will never qualify as "precise modelling".
but if not study as many images you can find from all angles.
True that. I don't know for sure, but I suspect the only available cross-section drawing of this a/c (and its sibling) remains the "OvalLite"-cabin section advertisement from which I derived the basic fuselage of my 300. I have that model on the shelf for years now and I remember modelling that thing was mostly about watching images from every angle, and then pushing vertices accordingly. In Blender, you can create (Shift+A) cameras from a specific angle and, knowing the settings of the real world camera the photo was taken with (flickr, airliners.net & Co often have this data), superimpose the photo on the mesh while you work from the Camera view (just like background 3-view drawings in Ortho front, side or top view).
the landing gear bays it's a big problem for me. How can I make it? Modeling a new cylinder?
If you mean the wing-to-fuselage fairing, and if you have basic knowledge of NURBS curves and surfaces in Blender, create a surface, shape it as closely to the plane's belly as possible, convert to Mesh (Alt+C) and start adjusting loops.
Or you can indeed create a cylinder with enough sections, orient its axis along the fuselage, delete the top half and adjust the loops to fit the shape.
If you want to go the absolute b a s i c way, go to front view, add (Shift+A) a circle of at least 64 verts, delete the top half and make the remaining vertices follow the outline of the belly from the 3-view drawing (front). Go to side view and translate (G) the loop to where the lowermost point of the belly is. Now extrude (E) in whichever direction you want to start with and adjust the new vertices to fit the shape, using a combination of 3-view drawings in the background, photos and guessing. Switch from front to side view as often as needed...
You picked a nice challenge! It's a beautiful aircraft.