The sound came out of nowhere.

It began as an unnerving high-pitched hum, like a mosquito flying around a dark room at bedtime, and it grew louder as it drew closer. A family lounging in a backyard pool in Richardson, Texas, just north of Dallas, couldn’t help but notice it.

“There’s another one coming this way!” shouted Eric Anderson from his pool float, pointing to an area above a tall line of trees. His 9-year-old daughter, Isla, removed her swim goggles for a clearer look at what could be a scene from science fiction.

Within a minute or so, an 83-pound drone with six blue arms and whirring propellers appeared in a corner of the sky before zipping over nearby houses, looking like a cross between a mechanical dragonfly and an X-wing fighter, and buzzing like a battalion of bees.

The drone’s mission: delivering an Amazon package.

The single cardboard box of cargo held a maximum of five pounds and was loaded into the drone at Amazon’s distribution center less than a mile from the backyard pool. It could have been bound for anywhere within a seven-and-a-half-mile radius of takeoff.

Richardson city officials gave Amazon zoning approval to start the drone delivery service in December as part of the company’s nascent Prime Air program. It promises customers tens of thousands of items — nail trimmers for pets, ankle bracelets, handcrafted, razor-tipped bread lames for scoring sourdough — in as little as 30 minutes. The delivery could be free or cost up to $4.99.