In retrospect, signs of Amazon’s secret tests were hidden in plain sight.

There was the warning to pilots that unmanned aircraft would be flying in the area, about an hour north of London, until early October; the uncharacteristically fast cellphone reception in such a remote area — a must when processing drone data; and the growing list of jobs and openings at Amazon’s research and development site in Cambridge related to Prime Air, the company’s ambitious plan to use drones for everyday deliveries.

Image A drone flying over Amazon’s testing site in Cambridgeshire, England. Credit... Andrew Testa for The New York Times

Amazon is not alone, however, as other companies conduct drone trials around the world.

In New Zealand, Domino’s Pizza is testing drones to ferry fast food across the country. Google is offering burrito orders delivered by drone in Virginia. JD.com, the Chinese e-commerce giant, already has a fleet of drones flying autonomously for a maximum of 15 miles round-trip, to reach rural communities at a fifth of the cost of traditional trucks (though a person still takes the package on the last leg of its journey to the recipient).

In Britain, Amazon is working with local authorities to test several aspects of drone technology like piloting the machines beyond the line of sight of operators, a practice still outlawed in the United States.

Regulators here first authorized the commercial use of drones in 2010 — years before the Federal Aviation Authority eased its restrictions on remotely piloted aircraft in June. Amazon settled on Britain after the United States initially denied it approval for such tests.

At the site in the Cambridge countryside, and a similar facility in Canada, Amazon is likely to be working on the drone’s sensors and other improvements needed for its daily use.