The U.S. Air Force is considering fielding a variant of its next-generation bomber that could collect intelligence, surveillance and reconnaissance (ISR) undetected behind enemy lines.

Air Force Secretary Michael Wynne says the service is on “a quest to have long-range reconnaissance.” He says that an unmanned version of the bomber, which is expected to be fielded in 2018, would be a strong candidate for this mission.

Penetrating ISR has been lacking since the retirement of the SR-71 Blackbird in the 1990s. The high flying U-2 and Global Hawk unmanned aerial vehicle can collect a variety of intelligence, but their vulnerability to detection forces them to operate at standoff ranges.

The U-2 carries more sophisticated sensors but is limited to about 12 hours of flight time due to the limitations of an onboard pilot. The Global Hawk, which has flown missions lasting longer than a day, has not yet proven its ability to collect signals intelligence.

The bomber platform is expected to be subsonic, highly stealthy and carry between 28,000-40,000 pounds of payload. An ISR version could operate undetected in airspace defended by the most advanced double-digit surface-to-air-missile systems.

Service officials still expect to keep a pilot in the bomber cockpit for those variants certified to deliver nuclear weapons.

Northrop Grumman and a Boeing/Lockheed Martin team are vying for the work. The Air Force expects to include the first significant funding for the bomber in its fiscal 2010 budget, which is now being crafted in the Pentagon.

Already though, many of the contributing technologies are being funded through classified accounts, Wynne says. The challenge for the system will be to integrate them together.

One technology hurdle, however, will be to reduce the infrared signature emitted from the bomber’s propulsion system. It remains unclear whether systems with serpentine exhaust and a reduced signature will be ready for the initial fielding of the bomber in 2018.

The Air Force was notionally discussing a buy of 100 bombers, and that number could increase with the addition of an IRS variant.