Recently we've seen lots of this small fly in our house in Michigan. They are smaller than regular houseflies and don't have the distinctive green/blue sheen that houseflies have. I assume they bite, because at least once I've killed one and it spewed red (which I assume was blood), although I haven't actually been bitten by one to my knowledge.

The fly is about 8mm in length. They tend to sit on walls or ceilings without flying a lot, and they're not difficult to kill, i.e., they don't immediately fly off when approached unlike a regular house fly.

Is it possibly a [cluster fly](https://en.wikipedia.org/wiki/Pollenia_rudis)?
 

Here are two images. The first image is a live fly, the second is dead which is why the wings aren't flat to the body. 

[![fly 1][1]][1]
[![fly 2][2]][2]


  [1]: https://i.sstatic.net/pp5HQ.jpg
  [2]: https://i.sstatic.net/ffMOd.jpg