The Airborne Fortress is the very first giant aircraft in the history of the Ace combat series. It was featured in the game Air Combat, the very first installment of the series.
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Little is known about the Airborne Fortress, expect that, based on its apperance, it looks like a mix between the XB-0 Hresvelgr and the Gleipnir. It's propelled by four engines (which contains 6-8 engines each). It has very few armaments of AA guns and SAM's - although its lightly armed, the Airborne Fortress is still a very dangerous target to deal with.
How did this advanced piece of aerospace technology got to terrorist hands is unknown at best.