Southwest Airlines' launch order for the 737 Max will guide the aircraft's development, including which variant is built and tested first.
"Southwest has the flexibility to choose from 737-7s and 737-8s so it's their decision on which will be first," said Boeing of the 150-aircraft launch order.
In its role as launch customer, Southwest - a staunch advocate of minimum change technologies to optimise products and innovate its operational processes - will guide the development of the new 737 variant, which includes the re-engined 737-7, -8 and -9.
"We tried to stay with a commonality theme," said Brian Hirshman, Southwest senior vice president of technical operations. "We wanted this to be as much commonality as we can compared to the 737NG. Of course, we wanted to improve upon efficiency and productivity around the airplane, so we're been working closely with Boeing and it's every much an iterative process. Our intention is to play a very important part in the development of the airplane right up through entry into service."
Boeing hopes to minimise the changes between the Next Generation 737 and the 737 Max preserving both the common pilot type rating and grandfathered US Federal Aviation Administration certification.
Boeing anticipates widespread structural strengthening, as well as lengthening the 737 Max's nose landing gear up to 20cm (8in) to accommodate the larger 173cm (68in) engine fan, while developing a new pylon and fly-by-wire spoiler system for the variant.
The first production 737 Max, which will be built on the airframer's Renton, Washington final assembly line, will be delivered to Southwest in 2017, the first of four the airline expects to receive that year.
Boeing first launched its Next Generation 737 family with its -700 model for Southwest, though throughout its production run the larger -800 has become the type's most popular variant, suggesting that Boeing may opt to develop the CFM International Leap-1B-powered 737-8 first.