Sky Fortress is part of Just Cause 3's Air, Land & Sea Expansion Pass, a $24.99 add-on that includes three expansions. As you may have guessed, Sky Fortress constitutes the "air" part of the package. It will bring "new missions, features and other exciting surprises to the skies of Medici," according to Avalanche.
Avalanche did not confirm a release window for Sky Fortress, but said it is currently in the playtesting phase of development on the DLC. As publisher Square Enix announced previously, the Air, Land & Sea add-ons will each be available one week early to owners of the Expansion Pass.
A separate part of the Avalanche team is continuing to work on fixing bugs and improving performance in Just Cause 3, according to the studio. The first update, which Avalanche released in mid-December, addressed stability issues and long load times. The company expects to roll out the game's next patch later this month.
For more on Just Cause 3, read our review.