"With no real primary scholarship on the NWT committee, I'm guessing that the gang made use of the same tools you mentioned, concordances, lexicons, etc. Then they had to have dozens of translations to refer to. With said tools in hand (no pun intended), I'm guessing Franz & company proceded to cherry-pick the verse translations most favorable to the WT worldview. Hell, anyone with the time and desire could probably put together their own translation that way. You certainly wouldn't need any real knowledge of Ancient Near East languages, society, etc. "
This is also what I think happened. They wanted their own Bible 'translation', not just for doctrinal reasons, but for copyright reasons. They wanted a Bible they could publish freely in modern English. They wanted one that did not have the biases in translation they did not accept, especially in reference to the Trinity, and they wanted to add a few of their own. So, they looked at a few other translations and concordances, lexicons, etc. to cobble together their own.
The result is not the terrible translation some claim it to be. It is no better and no worse than many others. But of course, it does support their doctrinal position and that upsets many, even though other translators do the same thing.
I have to admit, I would never ever use it again because it is tainted just by the fact JWs compiled it.