If you fly, you've almost certainly found yourself packed into a Boeing 737. This workhorse of commercial aviation accounts for one of every three commercial flights, and there are around 2,000 of them in the air at any given time.

Every one of those planes rolled out of Boeing’s Renton Production Facility, where workers build a 737 in just nine days. The factory, near Seattle, pump them out at the rate of 42 per month, and Boeing claims the 1.1-million-square-foot facility is most efficient airplane factory in the world.

Boeing faces fierce competition from arch rival Airbus. The 737 is the best-selling jet ever, with more than 9,000 delivered since its introduction in 1967, but the Airbus A320 is no slouch. The company has delivered about 6,700 of them since the airliner entered service in 1984.

Strong demand for single-aisle jets from low cost carriers and domestic airlines in China means both companies are ramping up production. Airbus says it will crank out 60 planes a month by 2019.

Boeing plans to build 47 planes each month next year and ramp up to 57 in 2019. To find out how, WIRED visited the refurbished and enlarged factory in Renton to see just how a 737 goes together.