SEATTLE/SYDNEY (Reuters) - A new version of Boeing Co's BA.N 737 jetliner took off for the first time on Thursday, marking another step in Boeing's revamp of its best-selling product line that could see up to five new models introduced by 2020.

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Boeing’s 737 MAX 9, a fuel-efficient, long-range successor to the 737-900, made its initial flight, a test mission, about 50 years after the 737 first took to the skies on April 9, 1967, and follows the first flight of Boeing’s largest Dreamliner model, the 787-10, last month, from its factory in South Carolina.

The new models are coming to market just as demand for aircraft has slowed. Boeing has committed to making four new 737 versions and is considering a bigger version than the MAX 9, known as the 737 MAX 10X.

It could begin delivering that version in 2020 if airlines start ordering it this year, with China emerging as a promising market, said Michael Teal, 737 MAX chief project engineer and deputy program manager.

With the MAX 9 in the air, Boeing is turning more attention to the larger version. It began pre-marketing the plane this year but has yet to formally launch development of the jetliner, which would compete against the popular Airbus AIR.PA A321neo.

Teal said the design of the 737 MAX 10X would be firmed up by the end of this year and customers could receive the aircraft in 2020, depending on orders.

“We’ll determine (the delivery date) when we launch that program when the customers show the interest and they buy the airplane,” he said on a conference call with reporters.

CHINA

“The China market and airlines there are very interested in the (MAX) 9 and 10 depending on their needs,” said Teal. “We’re looking at putting together deals for the China market today on both the 9 and the 10.”

Boeing said in March it had approached India's SpiceJet SPJT.BO and Jet Airways JET.NS about the MAX 10X, which its marketing chief says would offer the best efficiency in that part of the market, a claim Airbus rejects.

However, the heads of two major aircraft leasing companies have voiced skepticism about the appeal of the MAX 10X and suggested it would eat into rentals of other MAX models.

The 737 MAX 10X would be 66 inches (167 centimeters) longer than the current largest family member, the 737 MAX 9, and add 12 seats. It would require longer landing gear as a result.

Teal said the landing gear was still in the development stage, with several concepts in prototype testing.

“We won’t hit the firm configuration on the gear and, really, the complete airplane until the end of this year,” he said. “All of the development tests are proving positive and we are well on our way to firming up that configuration and moving forward into production.”

The first 737 MAX 9 customer delivery is expected next year, while the first 737 MAX 8 should be delivered next month.

“We start with several airplanes in May, more in June and more in July and we start the ramp-up from there,” Teal said.