Thanks for the pic. The P-39 lines are still apparent, although the Model 4 sketch has a fair resemblance to the Consolidated fighters of the 30's. I'd like to think of it as a missing link in fighter evolution.
I'm all but certain that the doors over the nose were to access the armament. The panel lines under the rear canopy were to access the engine. On the following pictures of the XP-39, note the same panel lines on the nose and the air scoops on the side of the fuselage that are hinted at in the top view of the drawing. The claptrap on the belly is part of the turbocharger system that was deleted from the Y and production aircraft. Also shown are the clear plastic leading edge that housed a radio antenna, the higher canopy, and the prototype's landing gear door arrangement. These photos are from the U.S. National Archives.