By Conor Humphries

Seattle (Reuters) - Boeing Co <BA.N> is still studying whether to build a larger version of its best-selling 737 aircraft to counter runaway sales by rival Airbus <AIR.PA> of a large version of its competing plane, a senior executive said.

The company has not made a decision yet to build the larger jet, dubbed the 737 MAX 10, said Michael Teal, vice president and chief project engineer of the 737 MAX program.

"We're looking at various alternatives ... different varieties of engines, fuselage length," Teal said in an interview at Boeing's 737 production facility in Renton, Washington on Tuesday evening.

"It is being studied and it's just a determination if we find the right solution for the customer and the timing and how much change we would have to do to make that work," he said.

The Airbus <AIR.PA> A321neo can hold up to 240 passengers, compared with a maximum of 220 on the planned 737 MAX 9.

The MAX 10 would be developed after its other MAX variants. It would enter service after the smallest version, the MAX 7, comes out in 2019, Teal said.

Boeing is studying a wide range of options for the larger planes, including using the engines designed for a single-aisle plane being developed by China's COMAC.

Boeing already has decided to enlarge the MAX 7, adding 12 seats. The MAX 7 will have a range of around 3,500 to 3,600 nautical miles, longer than the MAX 8, he said.

Flight testing of the initial 737 MAX 8 planes is on schedule and expected to conclude late this year or early next year, allowing the first 737 MAXs to go to customers in 2017.

"We're gonna meet that [deadline]," Teal said. "It's going pretty well. All four [test] airplanes are flying."

Boeing has already built 10 MAX planes and the 11th is in production now. The company is building the 787 MAX 8 first, with delivery in 2017, followed by the MAX 9 in 2018 and the MAX 7 in 2019.

Teal said he didn't have concerns about supply chain problems as 737 production rates increase and the Max comes into service.

Most of its 737 production in 2017 will be 737 NG (next generation) models, making more MAX models by the end of 2018, he said.

(Writing by Alwyn Scott; Editing by Jeffrey Benkoe)