The cockpit interior doesn’t require extensive detail, but adding it restores the fuselage model to being a single continuous surface.
Begin modeling the interior of the cockpit:
 zoom and 
 pan to get a better view of the top of the cockpit.
The P-47 fuselage now has a cockpit-shaped hole in it. To make the model a continuous surface again, you will add faces to form the interior of the hole.
 Polygon Modeling panel, click 
 (Border).
            
 Click the edge of the cockpit to select all of the border edges.
Shift+move the border downward to clone it and create a rim for the cockpit.
Shift+scale the new border uniformly, to make the cockpit interior wider than its rim.
               Watch all four viewports when you do this step: You don’t want the sides of the cockpit interior to extend beyond the outside of the fuselage!

Shift+move the cockpit border down, until the new border is just above the level of the wings.
 Align panel, click 
 (Align Y) to make the new border horizontal.
 zoom and 
 pan so you can see most of the opening at the bottom of the cockpit.
 Geometry (All) panel, click 
 (Cap Poly).
               3ds Max creates a polygon to cap the open border.

 Zoom, 
 pan, and 
 orbit the Perspective viewport so you can see all of the floor of the cockpit.
The polygon you just created is large and multisided, so you need to subdivide it into quadrangular polygons.
 Polygon Modeling panel, click 
 (Vertex).
            
 click and Ctrl+click to select the two vertices where the forward edge loop stops at the cockpit floor, then on the ribbon 
 Loops rollout, click 
 (Connect).

 Polygon Modeling panel, click 
 (Edge).
            
 Click an empty part of the viewport to deselect any edges that are automatically selected, then click and Ctrl+click to select the two edges you just created.
 Loops rollout, click 
 (Connect).
 Edit panel 
 (Cut) tool to connect the vertex at the front floor level of the cockpit to the one at the middle of the leading floor-level
               loop. Use the Cut tool again to connect the vertex at the middle of the trailing flor-level loop to the one in the middle
               of the back floor level of the cockpit.
Now the polygons that form the interior of the cockpit are all quadrangular, and follow the overall pattern of the polygons that form the exterior of the fuselage.
 Zoom, 
 pan, and 
 orbit the Perspective viewport so you have a more edge-on view of the cockpit.
            
 Click to select one of the vertical edges along the side of the cockpit interior.
 Modify Selection panel, click 
 (Ring).
               3ds Max selects all the vertical edges, in a ring around the inside of the cockpit.

 Loops panel, click 
 (Connect).
               3ds Max connects the edges with a new horizontal loop of edges.

 Edit panel, activate 
 (Constrain To Edge).
            
 Move the new edge loop up a bit.
               
 Edit panel, activate 
 (Constrain To None).
            
 Zoom and 
 orbit the Perspective viewport so you can see the polygons at the front of the cockpit interior.
 (Polygon) sub-object level.
            
 Click and Ctrl+click to select the two polygons at the bottom of the front wall of the cockpit interior.
 Polygons panel, turn on 
 (Extrude). Drag to extrude the polygons forward, providing leg room for a pilot. (The two polygons above the leg room but
               below the cockpit rim would be the location of the instrument panel.)
 (Polygon) sub-object level.