The Navy's future robotic air wing is taking shape. On Tuesday, the sailing branch announced that it will pay four companies to hand over the technical specs for their various designs for the Unmanned Carrier-Launched Airborne Surveillance and Strike system (UCLASS), a stealthy, jet-powered killer drone meant to operate off an aircraft carrier and fly alongside the latest manned fighters. The move clears the way for the Navy to pick one of the four designs to form the backbone of one of the most ambitious drone efforts ever. Boeing, General Atomics, Lockheed Martin and Northrop Grumman all "have credible, existing, comprehensive, UCLASS design solutions," the Navy said in its formal announcement. The sea service wants each company to produce a working prototype for a competitive fly-off "to support fielding a UCLASS capability within three to six years," likely to occur in mid-2014. The Air Force is also considering buying whichever 'bot the Navy picks -- finally giving the Pentagon the stealthy, jet-powered armed drone of its dreams. With potentially billions of dollars at stake -- not to mention the chance to shape the military's future aerial arsenal -- all four companies began working on their designs years ago. When the Navy picks its winner, probably in 2015, one of the following four designs, each with its unique strengths, will become America's next killer drone. X-47B Of the four killer drone candidates, Northrop Grumman's X-47B (pictured above) is the clear frontrunner. Since 2011, the 62-foot-wide batwing 'bot has been flying tests for the Navy under a $2 billion precursor program to UCLASS, one meant to prove that high-performance drones can function on carrier flight decks. The Navy is already familiar, even comfortable, with the X-47B. Colloquially, it calls the 'bot the "Iron Raven," a nod to the old Grumman Iron Works and how it seems to have a raven's beak when viewed in profile. In the second week of May, the Navy plans to launch the X-47B from the deck of the USS George H.W. Bush in the Atlantic, marking the first time a robot has ever taken flight from a carrier at sea. But the X-47B, a descendent of an earlier killer drone with its roots in the late 1990s, is possibly the least stealthy of the competitors, owing to Northrop's decision to build the drone big, thick and tough. Those qualities help it survive forceful carrier landings, but also make it a big target for enemy radars. Navy Capt. Jamie Engdahl, manager of the drone test program, described it as "low-observable relevant," a careful choice of words copping to the X-47B's relative lack of stealth. Spencer Ackerman contributed reporting. Photo: Navy

Phantom Ray A decade ago, Boeing's X-45A drone battled Northrop Grumman's X-47A, a smaller version of the X-47B, in a competition to provide the Navy and Air Force with new killer drones. But the Air Force unexpectedly backed out. The Navy rescued the X-47 for the UCLASS precursor test program, but the X-45 was left without a sponsor -- a shocking move to many drone developers, who praised the X-45's superior software. Undeterred, Boeing quietly used its own funds to redesign, enlarge and improve the X-45. The resulting Phantom Ray, 50 feet from wingtip to wingtip, flew for the first time two years ago, showing off a thinner and potentially stealthier shape than its Northrop rival. (Some in the Navy have cheekily taken to calling the Phantom Ray the 'X-45C.') Boeing has declined to comment on the Phantom Ray's software-based control system, but if it's at all based on the X-45A's cutting-edge code, the new drone's real strengths could be under the skin. Photo: Boeing

Sea Avenger General Atomics cut its teeth manufacturing the mainstay Predator and Reaper drones for the Air Force and CIA. The jet-powered Avenger, unveiled in 2009, was intended to be a faster, stealthier follow-on. But the Air Force acquired just a handful of Avengers for testing, seemingly leaving the new drone in development limbo until the Navy's UCLASS initiative offered a way forward. The 66-foot-wingspan Sea Avenger, as the carrier-compatible version is called, doesn't have the long lineage of the Northrop and Boeing drones, but it does boast one big advantage. General Atomics is planning to fit the new 'bot with advanced sensors, including a compact, ground-scanning radar and the same high-resolution cameras carried by the F-35 Joint Strike Fighter. Photo: General Atomics