United Continental Holdings Inc said it will buy 100 Boeing 737 MAX 9 aircraft and fifty 737-900ER planes in a deal that Boeing valued at $14.7 billion at list prices.

United Continental, the holding company that owns United Airlines, the world’s largest carrier, said it will begin taking delivery of the 737 MAX 9 planes in 2018.

The additional order for fifty 737s, which will be delivered from late 2013, reflects efforts by both Airbus and Boeing to maintain production of their current products and ensure a smooth transition to newer models from 2017.

“These new aircraft will allow United to replace older, less-efficient aircraft to reduce fuel and operating costs, United said in a statement.