Each flyable aircraft in the game has a small horizontal bar underneath the aircraft name on the aircraft selection box. This is the 'Kill Bar'. In order to get the other variants of certain aircraft that are part of a family, you'll need to fill this all the way up by destroying enemy targets. You must gain a certain amount of points (cumulative) with an aircraft in the family of the plane you want. The amount of points you need to earn will differ with each aircraft. For the stronger and better planes, you will need to earn more points to unlock it's new variant. After filling up the Kill Bar of the desired plane, the next aircraft in the aircraft family will become available for purchase at the 'Buy' screen. Kill Bars can be filled up in both Campaign and Free Mission mode.The aircraft that are the first variants of their families are obtained by completing certain missions in the game, and will become available after completing them.NOTE: Aircraft that have only one variant in the aircraft family (ex. MiG-29A, MiG-1.44) will have their Kill Bars already filled up when you buy them, so you do not have to fill up their Kill Bars at all. The last plane in each family will also have its Kill Bar filled up already.