The U.S. Air Force's super-secret next generation bomber, which one day could fly unmanned sorties, has moved a step closer to the production line with the Pentagon asking top defense contractors to submit proposals.

Not much is known about the Long Range Strike-Bomber project, which has been run as a classified program since 2011. But the bat-winged stealth bombers would likely cost around $550 million each, and the Air Force hopes to contract for as many as 100. That would add up to as much as $55 billion -- for a fleet of fighter planes the likes of which the world has never seen. Still, the pricetag would be lower than the B-2 currently used.

“The LRS-B is a top modernization priority for the Air Force. It will be an adaptable and highly capable system based upon mature technology,” Air Force secretary Deborah Lee James told the U.S. Naval Institute.

The Air Force's request for proposals is aimed at finding a prime contractor by spring of next year. Northrop Grumman and a Boeing/Lockheed Martin team are the natural choices to build the new bomber.

The Air Force was tight-lipped about the plane, but it is believed that it will be a very long-range subsonic aircraft with broadband stealth capability designed to evade low-frequency radars as effectively as high frequency. The planes will also be certified to carry nuclear weapons and are being designed as part of a Long Range Strike family of systems—and not as an aircraft designed to penetrate into the most heavily defended airspace all by itself, USNI reported.

The LRS-B was originally intended to be an “optionally manned” bomber, but the Air Force has indicated the aircraft will be at least initially manned. It is believed the planes will be larger, carry a bigger payload and have much longer range than the current generation of bombers.

According to Congressional Research Service analyst Jeremiah Gertler, who studied planned funding trendlines in a recent report and determined the Air Force has already done most of the development work on the bomber already. It is possible it has already even built prototypes, in the classified “black” budget, according to BreakingDefense.com.

“My idea is that something related to LRS-B might be already flying,” David Cenciotti, who operates the popular blog The Aviationist, told FoxNews.com. “It could be a technological demonstrator, a prototype or even a production plane.”

Cenciotti said an amateur photographer in Wichita, Kans., is believed to have photographed the plane .

“Even if it may seem far fetched, I think the one over Kansas could be the next generation LRSB,” Cenciotti said. “The reason why the Pentagon has eventually decided to fly the aircraft in daylight (hence exposing it to observers) is still unknown: maybe to distract from something else, or simply to flex muscles with Russia amid growing tensions over Ukraine.”

Even the bases from where these aircraft could be launched and recovered remain a mystery, according to Cenciotti.

He said Nevada's Groom Lake Air Force facility, also known as Area 51, or Whiteman Air Force Base, home of the B-2 fleet and only about 250 miles east of Wichita, would make the most sense.