Boeing has tentatively scheduled the first flight of the 737 MAX for Friday, a key milestone for the best-selling aircraft.

Boeing had said it planned to make the flight in the first quarter of 2016. Like all first flights, the 737 MAX takeoff could be delayed by weather or other conditions, Boeing said.

The Chicago-based company said it finished key development steps, readying the plane for its first takeoff. It is due to be delivered to airlines starting in 2017.

The first 737 MAX flight comes 17 months after Airbus first flew its rival A320neo equipped with Pratt & Whitney geared turbofan engines, and nine months after the A320neo equipped with CFM engines first flew.