In War Wings there are several missions that you can complete as you play the game. Missions are basically Achievements that you acquire by completing them. Make sure you always check to see what missions are attainable in your current level and focus on trying to complete them as soon as possible as doing so will reward you with a decent amount of silver which you can use to upgrade different areas of your plane such as increasing your firepower or strengthening your armour to make your it stronger.