If you grow any purple flowers in eastern US, you have probably seen these and maybe confused them with large bumble bees. They look similar to a bumble but have a shiny hairless black abdomen. http://en.wikipedia.org/wiki/Eastern_carpenter_bee
Here in Michigan, I usually see these things in mid summer. They love Vitex Agnus and I often see a half dozen at a time on the vitex. The drones are very territorial. I wore a bright florescent orange shirt to a plant nursery one time and was chased around all day by a male carpenter bee.
I have a lot of bumble bees here, but by mid summer the bumble bees I see are usually on the smaller side (worker bumbles). Queen bumble bees are indeed large, but you really only see them in the early spring. Once the queen bumble raises brood, those workers do all the foraging the rest of the summer. The queen stays in the nest.
The carpenter bees in these photos must be raising brood. I see they’re hauling around a lot of pollen.