He will be great, and will be called the Son of the Most High. The Lord God will give him the throne of his father, David,
Inherent in these words of the angel is also the fact of Mary's descent from David.  Mary herself being the only physical link that Jesus ever had with that monarch.  Joseph, the husband of Mary, was also the direct heir to the Davidic throne, through Solomon, thus making Jesus the legal heir of David, as well as his fleshly descendent.