United Airlines said it would buy 25 single-aisle aircraft from Boeing Co in addition to the 40 it had ordered earlier.
Reuters reported in February that United Airlines had placed a new order for the 126-seat Boeing 737-700 aircraft, which would be worth just over $2 billion at list prices.
United Airlines, a unit of United Continental Holdings Inc, did not disclose the financial details of the new order on Tuesday, but said it would start taking delivery from 2017-end.
The company will also convert a previous order for 27 Boeing 787 aircraft into an order for four 777-300ERs and five 787-9s.
The delivery of the earlier order was expected from 2020 and the new order will be delivered from 2017 onwards.
