Having established himself as a favourite with the Medici family, there was little to prevent Fra Angelico from becoming one of the key figures in the Early Renaissance. This particular fresco is dated loosely at 1437-1446 and would not have taken nine years by itself, reflecting the difficulty that exists in dating art from the 15th century accurately.
Fra Angelico would use his highly skilled assistants to help out on larger pieces but this work would not have required anything additional to the talent and experience of this wonderful artist. Indeed, at this point in his career he would have been at his pinnacle, both technically and also in terms of financial and political backing.
Beyond the various depictions of the Annunciation by Fra Angelico, this theme has been covered by other famous names such as Leonardo da Vinci with his version of the Annunciation from 1472-1475. There were also stunning productions from master such as Titian and Peter Paul Rubens.