This is a species of Robber Fly (insect family Asilidae), a group of flies that prey on other insects (both as adults and as larvae). Adults catch other insects in flight, and often land to consume them. They are considered beneficial in garden settings, and Maryland is home to about 100 species that range in size from this individual (one of the larger species) to much smaller. The species you photographed might be an Eastern Hammertail (a male, given the abdomen shape), though we're not positive since a few species look alike.

Miri