Start by making a box. Be sure to set the origin at the bottom. You can use any settings, but these are the settings being used for this tutorial:

Once that is finished, create a tube using one section, in the Z axis. Make the length of the tube the same as the depth of the box, make the radius exactly one half the width. Put the Y centre position at the same level as the top of the box.

The front view of the object should look like this:

Switch to polygon mode, and delete all the polygons which hang down below the top of the box:

Select the box, go to polygon mode and delete the one on top.

If you look in the 3D view, it should now resemble the following:

Switch to point mode, you will see that the points which made up the already deleted polygons are still in place.

To get rid of them, choose Delete Orphaned Points from the Parts menu.

Now, in the side view, select all the points along one side. Press Ctrl-P to create a polygon out of these points.

If the polygon is pointing in the wrong direction like it is below, be sure it is the one highlighted in white, then press the "F" key. Repeat this action with the other side.

The 3D view should now look like the following.

Now create a new box, smaller than the first, and make sure the edge of it lines up perfectly with the original box. Here are the settings you could use:

And here is what it should look like.

Now select the two boxes,

and press the J key to join them together.

Switch to points mode, and select the four points farthest away

And delete them so that all the points now line up

In polygon mode, delete the two polygons that are on the same side as the sections just deleted.

The 3D view should now look like this:

Now comes the tricky part. Select the four points shown. But be sure they are selected in clockwise or counter-clockwise order. Your first attempt may end up selecting them in the wrong order.

Press Ctrl-P to create a new polygon. If it's pointing in the wrong direction, press the F key to flip it. If the polygon looks deformed, the points were probably selected in the wrong order. Delete it and try again. The 3D view should look like this:

Repeat the action until the model looks like this:

Now select both parts

And press the J key to join them together.

Now copy the part, and paste it back on top of itself. Switch to polygon mode, select the Polygon menu then Flip All/Selected.

Here is the final project:

You can now add textures, and scale the parts in the right directions until the hangar is the size you want. It might be easiest if you join the parts first.

Before you can take your plane inside, go to Properties under the File menu, and make sure crash detection is turned off.