Yep, that was "First Flight", IMO one of the memorable episodes of ENT.
I stumbled over it on Memory Alpha
, who linked to an interview with Mr.Braga by trekmovie.com
In comment #124 Braga wrote: You know, Rick’s initial idea was to play the first season on Earth as they were building the very first warp ship. But that was way too off-concept, I think, for the studio.