Amazon has won approval from the Federal Aviation Administration to begin testing delivery drones in the United States.

Last month, Amazon was granted a similar certificate from the FAA, but because that request spent six months in regulatory limbo, the prototype drone approved by federal regulators had become obsolete. In a letter issued Wednesday (April 8) and addressed to Paul Misener, Amazon's vice president for global public policy, the FAA approved the outdoor testing of Amazon's delivery drones.

"This letter is to inform you that we have granted your request for exemption," John Duncan, the FAA's director of flight standards service, wrote in the document, which was posted on the agency's website. "The exemption would allow the petitioner to operate an unmanned aircraft system (UAS) to conduct outdoor research and development testing." [5 Surprising Ways Drones Could Be Used in the Future]

The terms of the FAA's approval state that Amazon drone operators can conduct test flights at altitudes of up to 400 feet (122 meters). Federal regulators also said the flying bots must not exceed 100 mph (160 km/h), and must remain within the pilot's line of sight, according to the letter.

Amazon is developing a same-day delivery service, dubbed Prime Air, that would use octocopter drones to airlift packages weighing up to 5 lbs. (2.3 kilograms) to customers within a 10-mile (16 kilometers) radius of an Amazon warehouse.

Jeff Bezos, founder and CEO of the online retail giant, announced his vision for Prime Air in 2013, saying drones will be able to transport packages from the company's warehouses to shoppers' front doors in 30 minutes or less.

"We're pleased the FAA has granted our petition for this stage of R&D experimentation, and we look forward to working with the agency for permission to deliver Prime Air service to customers in the United States safely and soon," Misener told Engadget.

In February, the FAA outlined a set of rules to govern the burgeoning commercial drone industry in the United States. The much-anticipated announcement proposed that drones weighing up to 55 lbs. (25 kg) could be flown at speeds of up to 100 mph and at altitudes of up to 500 feet (150 m).

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