KEY POINTS
  • United Airlines last week pulled the Boeing 737 Max from its schedules through early January.
  • The planes have been grounded since mid-March after two crashes.
A United Airlines airplane takes off at San Francisco International Airport.

United Airlines CEO Oscar Munoz said Wednesday that a firm timeline for the return of the Boeing 737 Max, grounded since March after two fatal crashes, remains unknown.

"No one knows," Munoz said in an interview with CNBC's "Squawk Box." "We've been doing this for seven months. The important part is that it's returned safely."