The Boeing 737 Max 8 on Thursday gained certification from the U.S. Federal Aviation Administration (FAA), clearing it for service with operators including Southwest Airlines and Norwegian Air Shuttle. Boeing said it has entered the final stages of preparing for the first Max for delivery to customers “in the coming months.”

The Max 8’s certification program began barely more than one year ago with four airplanes.

The first of potentially five variants of the new 737 Max family, the Max 8 seats as many as 189 passengers in its highest density configuration and 162 in a dual-class layout, placing it where Boeing calls the heart of the single-aisle market. Powered by CFM Leap-1B turbofans, it provides a 14 percent reduction in fuel burn and CO₂ emissions compared with the 737-800NG, according to Boeing.

Plans call for entry into service of the next largest airplane in the Max family—the recently unveiled Max 9—in 2018, followed by the smaller Max 7 and higher capacity Max 200 in 2019. Boeing on March 7 rolled out the Max 9 from its factory in Renton, Washington, ahead of anticipated first flight in April.

Meanwhile, studies continue into another possible stretch of the Max 9 called the Max 10X, a product ostensibly designed to compete with Airbus’s popular A321neoLR.

Still not officially launched, the Max 10 could reach firm configuration next year, in time for certification in 2020, according to Boeing 737 Max program vice president and general manager Keith Leverkuhn.

The 737 Max has become the fastest selling airplane in Boeing history, accumulating more than 3,600 orders from 83 customers. Its direct competitor, the Airbus A320neo, has collected firm orders for 5,063 from some 90 customers.