I do not think this is a fly. This looks like a black wasp, and we have these near where I live. The adult females are vicious when you get near their nest and I am fairly sure they bite, not sting. Someone I knew was bitten by one of these and they don't let go at any cost; it took some of my friend's skin off with it. About that comment that they would rather walk or fly, I agree. From what I've seen, they have been walking. I saw one on a boat and it was just walking across the deck.
In support of that information, that insect has the body setup of a wasp. Flies have a more put-together, barrel-shaped body. Wasps are more aerodynamic and their body segments are more spread out, like the black wasp specimen you posted of above.