Boeing and United Airlines announced the carrier ordered 24 additional 737 MAX jets and four more 777-300ER (Extended Range) airplanes last year.

The $4.5 billion order, according to list prices, was booked as unidentified on Boeing’s Orders & Deliveries website. The Chicago-based airline has steadily placed new orders for the two Boeing jets to serve its large domestic and international network.

The MAX builds on the 737’s industry-leading performance and reliability by offering operators more range and 14-percent better fuel efficiency compared to today’s airplanes and 20 percent more than the airplanes it replaces. The MAX achieves the improved performance thanks to the CFM International LEAP-1B engines, Advanced Technology winglets, and other airframe enhancements.

The 777 has become the best-selling twin-aisle jet family in history, earning more than 2,000 orders. In fact, United’s order pushed the program over the 2,000 threshold in December.