Projected mantels are designed to wrap around and enclose traditional firebox openings. The following are pictures of a projected mantel , the first illustrating before the mantel application, the second showing the finished illustration. Note that the marbel facing projects out from the wall about 6" :

Most pre-fabricated firebox installations are now done flush to the wall, to maximize space in the room. If you wish to have a projected look, (and therefore deeper mantel) the firebox can be extended out, or the stone or brick facing can be extended.

Below is a flush-mount firebox: