The NWT is not the only Bible to phrase it this way:
1808: “and the word was a god.” The New Testament in an Improved Version, Upon the Basis of Archbishop Newcome’s New Translation: With a Corrected Text.
1864: “and a god was the word.” The Emphatic Diaglott, interlinear reading, by Benjamin Wilson.
1928: “and the Word was a divine being.” La Bible du Centenaire, L’Evangile selon Jean, by Maurice Goguel.
1935: “and the Word was divine.” The Bible—An American Translation, by J. M. P. Smith and E. J. Goodspeed.
1946: “and of a divine kind was the Word.” Das Neue Testament, by Ludwig Thimme.
1958: “and the Word was a God.” The New Testament, by James L. Tomanek.
1975: “and a god (or, of a divine kind) was the Word.” Das Evangelium nach Johannes, by Siegfried Schulz.
1978: “and godlike kind was the Logos.” Das Evangelium nach Johannes, by Johannes Schneider.
Even Origen, the most knowledgeable of the early Christian Greek-speaking scholars, tells us that John 1:1c actually means "the Word [logos] was a god". - "Origen's Commentary on John," Book I, ch. 42 - Bk II, ch.3.
Jehovah's Witnesses have been criticized for allowing the indefinite article (a) at John 1:1c. However, the true fault lies with their critics. It is the other way around...the absence of the indefinite article at John 1:1c has been purposely mistranslated in most Trinitarian-produced Bibles to fit their doctrine that Jesus is God. (See Is Jesus God?)
For much more, see
If Jesus is not God, how can he be a god?
NWT - John 1:1
The Coptic Language and John 1:1
How does the Coptic text render John 1:1?
John 1:1c - English translation: "The Word was a god."
What About Trinity "Proof Texts"? (Scroll down to "The Word Was God")
IF God is a trinity, then how is it that “the Word (Jesus) was WITH God”? (John 1:1)
DEFinite John 1:1c
QUAL ("Qualitative" John 1:1c)
John 1:1c Primer
SEPTGOD (John 1:1c and the Septuagint)
God and gods