Boeing cut production of its best-selling jet by 20 percent in April to 42 a month from 52 a month, weeks after it was grounded worldwide in the wake of two fatal crashes.

Boeing estimates the return to service will begin "early in the fourth quarter" and based on this forecast it plans to maintain production at 42 a month, rising to the original goal of 57 a month in 2020, Muilenburg told analysts.

He said Boeing would continue assess these plans, however, and added: "Should our estimate of the anticipated return to service change, we might need to consider possible further rate reductions or other options including a temporary shutdown of the MAX production."