The Men Who x The Men Whom

I already know that "Who" is a subject pronoun and "Whom" an object pronoun, but there's a specific sentence which is getting me "confused" about it.

Which one of these sentences is written in the right, or at least the more acceptable, manner:

"The men who were designated to build the whole hotel." or
"The men whom were designated to build the whole hotel"?

If you could also explain me why, I'd appreciate.

Thank you in advance