Air Berlin is to buy an extra 60 Boeing 737 aircraft, taking its order up to 85 planes costing $5.7bn (£2.9bn).
Boeing is currently enjoying a sales boom
The German budget airline, which in August had said it would buy 25 of the aircraft, is to take delivery between 2007 and 2014.
Its announcement is the latest sales boost for Boeing, coming a week after Korean Air ordered 25 planes from the US aircraft maker.
While Boeing is enjoying a sales boost, its main rival Airbus is struggling.
The European firm has been hit by difficulties surrounding its forthcoming A380 superjumbo, which is now running two years late.
The delay saw one Airbus customer, global delivery firm Fedex, cancel its order for 10 A380s, and instead pledge to buy 15 Boeing planes.