When Amazon.com Inc. CEO Jeff Bezos unveiled a plan in December to deliver packages by drones, it was widely viewed as a public-relations stunt during the holiday shopping season. Less than nine months later, Google Inc.'s similar drone initiative is landing in a different climate.

The changed atmosphere reflects Google's financial and lobbying heft, which should help overcome technical and regulatory obstacles. But it also reflects growing confidence in technology until now more often associated with missiles than packages,...