Sky Mercenaries (Steam key/Region free).

http://store.steampowered.com/app/336090/
An epic sky battle awaits you! Choose your pilot and fighter to join the ultimate 2D S.H.M.U.P action ! Fight over 50 enemy types on 8 worlds in over 32 story missions and
unlimited random content. Be prepared for the overpowering enemy fleet and challenging bossfights. Upgrade your fighters, drones and pilots the way you like to play them. Get powerful and permanent visible upgrades, armor and different types of weapons. Level up your pilots and assign skill points in 3 different skill trees to unlock a variety of abilities like auto-shield, critical buffs and more. Join the Mercenaries now!