That is the bet that multiple defense contractors are placing, anyway, as they race to build unmanned aircraft that can pass evolving airworthiness certifications and replace police helicopters. And if that bet pays off, it will radically transform the way cities, citizens, and law enforcement interact.

There's a reason big drones like the General Atomics Reaper aren't already flying over the United States. The federal rules that govern aircraft in U.S. airspace are much stricter than those that cover U.S. military drones overseas. Many of the Federal Aviation Authority's regulations were drafted for manned aircraft, long before unmanned flight across the United States was even a possibility. Now the FAA is working with the private sector to update its rules for the age of ubiquitous unmanned flight, and that will open the floodgates.

"The market won't exist until the regulations exist," said Matthew Scassero, director of the University of Maryland Unmanned Aircraft Systems Test Site. "The FAA was a little slow in coming around to the realization that we needed to get those in place."