After many, many, many tries at stealing the jet but getting destroyed by the tanks and sometimes by the army, i stumbled on how to do this with stealth.
Along the sides of the entrances to the fort, there are small but wide and deep encaves big enough for 1 vehicle to drive along. This gives protection from fire.
Now once you enter the base, you are already seen and alerted to. You get about 4 - 5 stars automatically. If you use these trenches i just mentioned on the sides of the entrances to the forts to drive along, you can stay out of sight very easily. In fact if long enough, your wanted level will disappear and now you can go about getting whichever vehicle you wish without alerts.
* This has only been tested and proven if not seen by the enemies.
* This hasnt been proved if you kill a soldier silently.
Thought i would share.
Edit: I have successfully flew out and saved my lazer in my hangar.
Next goal is the Titan.