The U.S. Air Force is expanding its fleet of RQ-4 Global Hawk unmanned aircraft systems and has contracted Northrop Grumman for the drones.

The award -- firm-fixed-price contract -- is for three RQ-4s and worth $354 million.

The Global Hawk is a high-altitude, long-endurance unmanned aircraft with a speed of 357 miles per hour, a range of 8,700 nautical miles and a service ceiling of 60,000 feet. It operates multiple sensors simultaneously to gather intelligence, surveillance and reconnaissance (ISR) data.

Also covered by the contract are retrofit kits to add Airborne Signals Intelligence Payload sensors into two existing RQ-4s, which will give them a multi-intelligence capability. ASIP is an advanced ISR collection sensor also built by Northrop Grumman.

The U.S. Air Force currently operates a fleet of 34 aircraft.

The new aircraft are Multi-INT models that carry sophisticated imaging and electronic signals sensors capable of collecting multiple types of intelligence from high altitudes for up to 32 hours, Northrop Grumman said, and are to be delivered by 2017.

"Northrop Grumman is proud that the RQ-4 Global Hawk has become an indispensable ISR resource for the Air Force," said Mick Jaggers, Global Hawk program manager, Northrop Grumman. "These new systems will provide further opportunities to keep our warfighters safe.

"Even as Global Hawk flight hours increase each year, we are striving to reduce the overall operating cost of the system for the Air Force. The Air Force's commitment to putting more Global Hawks in the air and our dedication to meeting affordability agreements for production and sustainment ensure that this vital asset will remain sustainable and viable for many years."