United Airlines has converted eight Boeing 737-900ER options to firm orders.
The Chicago-based carrier has 91 firm orders and 52 options for the Boeing narrowbody following the move, according to Flightglobal's Ascend Online database. The aircraft will be powered by CFM56-7 engines.
United plans to use the 737-900ER to replace its domestic Boeing 757-200 fleet and less fuel efficient aircraft.
The additional aircraft are reflected in Boeing's orders and deliveries.
United did not comment by press time.