The Federal Aviation Administration has said it expects airlines to follow CDC recommendations that everyone wear face coverings when in public, but the regulatory agency has stopped short of requiring them.
Some airlines already require crew members to wear masks, and in addition to now requiring passengers to do the same, some airlines are blocking out middle seats, spacing out passengers during boarding and taking other steps to ensure social distancing on flights. They've stepped up their cleaning and disinfecting of airplane cabins, as well.
JetBlue was the first U.S. airline to require passengers to wear face masks, effective on Monday, May 4. Delta and United will make masks mandatory on Monday, too. Frontier's requirement goes into effect May 8, and American will require passengers to wear masks as of May 11.