Moth Bee Hybrid Look-Alike?

Twice this Spring, a strange insect that I have never seen before has flown into my house. It has the wings of a moth and the body of a bee. Similar to a yellow hornet, but it has gray and black stripes. It has a very large stinger. The moth/bee is about 3 inches long and the wing span of about 5 to 6 inches. They seem to be very aggressive. A friend of mine was stung by one of these last summer. Does anyone have any idea about what this might be?