Were there seat-belts in passenger planes back in 1939?

Yes, there were seat belts in passenger planes before 1939. But as far as I can find out, they weren't universally fitted. When they were used, they were simple leather or webbing lap belts with quick release buckles - nobody ever used rope. The Ford Trimotor used wicker seats - some were rather like armchairs, while others were wicker with leather padding. Try Googling "Ford Trimotor Interior" for a variety of photos.

Seat belts weren't required by the law but some planes have them to make the passengers feel better, but I'm pretty sure the seat belts back then didn't do much, They were flimsy leather/fabric strips sewed into the set and have a belt buckle thing to secure it, most people couldn't use them anyway because they were all standard sized.

Don't kknow for sure what yr, but the first seat belts were just rope you tied around your waist