Boeing is recommending pilots undertake flight simulator training, as well as computer-based training, before its grounded 737 Max returns to service.
The manufacturer has previously insisted its computer-based training was adequate.
However, the new guidance comes amid unprecedented scrutiny on the manufacturer after two fatal crashes involving the 737 Max, killing 346 people. The aircraft has been grounded since the second in March last year.
Boeing said its new recommendation was based on changes made to the aircraft following the crashes and "test results".
It previously said pilots able to fly older 737s only needed a short computer course of about an hour to adapt to the 737 Max’s controls, avoiding extra training costs for airlines.
The US Federal Aviation Administration (FAA) last year OKed Boeing’s stance in a technical advisory, but this has come under more intense scrutiny.