Time Patrol

Time Patrol is a clone of the arcade hit Time Pilot. In it, you fly around destroying enemy ships based on the time period you are currently in, and then destroy a 'boss ship' to escape the current time period, and jettison into a tougher one in the future. You start in World War I, versus biplanes which are easy to destroy, and progress from there. Level 2 is biplanes but smarter, and Level 3 is helicopters, for example.