Wal-Mart Stores Inc. has opened a new front in its battle with Amazon.com Inc. The world’s largest retailer has applied for a U.S. patent for a floating warehouse that could make deliveries via drones, which would bring products from the aircraft down to shoppers’ homes. The blimp-style machine would fly at heights between 500 feet and 1,000 feet (as much as 305 meters), contain multiple launching bays, and be operated autonomously or by a remote human pilot.