United Continental (UAL.N), 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 100 MAX aircraft order, first reported by Reuters in May , will allow United to replace older, less-efficient aircraft to reduce fuel and operating costs, the company said in a statement.

Boeing (BA.N) said the combined order will allow it to exceed 10,000 orders overall for the 737 family. The 737 MAX is a new-engine variant of Boeing's Next-Generation 737.

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

United Continental said the 737-900ER models will replace older, less-efficient Boeing 757-200 aircraft flown domestically.

The new models are expected to burn up to 15 percent less fuel per seat than the aircraft they replace, the company said.

The new set of orders takes United's order book -- anticipated through 2022 -- to 272, including 50 Boeing 787 Dreamliners, the airline said.

United Continental will be the first North American carrier to fly the much-awaited light-weight, carbon-composite widebody Dreamliner.

United shares were down 5 percent at $23.57 in morning trade on the New York Stock Exchange. Boeing shares were little changed at $71.42. (Reporting by Bijoy Koyitty in Bangalore; Editing by Joyjeet Das and Don Sebastian)

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