Ah, I see, sorry, so the angel in Daniel was in fleshly form. But again, the JW doesn't believe that faithful angels had that ability stripped from them, only the ones who gave in to temptation pre-Flood. (That being said, I never hear JWs suggest that God's angels are taking material forms today.)
I guess caroline77 was asking about both the pre-Flood angels who descended, and the demons who were cast out around 1914. But the answer is the same, that the Society believes they cannot materialize. However, as you said, the Bible itself does not state this.
Btw, you can download the new NWT in PDF form from jw.org, if you can stand visiting there :)