Boeing has begun work on a next-generation single-aisle aircraft that could eventually replace the 737 Max planes.

The new narrow-body aircraft is in an early development stage, which includes interior design efforts and discussions with engine manufacturers; CEO Kelly Ortberg met with executives from Rolls-Royce in the UK earlier this year to discuss a new engine for the plane.

The company also appointed a new head of products within the commercial airplanes division, whose previous role involved developing a new type of aircraft.

This news comes amid growing concerns among investors and travelers about the safety of the 737 Max, following an incident last year where a piece detached mid-air from an Alaska Airlines plane.