Before taking on the Douay do a comparison of Genesis 3:15 with another version. Check it out:
Genesis 3:15 KJV
And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
Genesis 3:15 DRA
I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel.
Knox, honest man, compares all 3 versions (Hebrew, Septuagint, and Vulgate). He points out that, as a prophecy of the Incarnation itself, and according to all the early Church Fathers, the feminine form of the verb is to be preferred.