-In the mission builder, create a mission and click on "Add plane" if there is not already one on the runway. Right click on the plane icon, then change the aircraft type from the normal F-15E to any plane in the game (Su-27, MiG-29, 707, etc.) Note: The plane that is selected will still look like the F-15E on the inside. Press F8 to switch to the outside view to see the aircraft. Additionally, the plane will still perform like the F-15E, allowing you to do barrel rolls and loops in a 707.
-Go to make your own mission. Your aircraft should be listed. Delete what it says is you, then click "Add Aircraft". Select what you want to fly from the list. Choose any other information and then click the "Computer Controlled" box to change it to "Human Controlled" if it already is not listed as such. Add what you want and save it, then fly it in the missions menu.