Northrop Grumman has become the fourth and final contractor to win an award from the US Navy to modify a preliminary design for an unmanned stealth bomber into an aerial tanker.

The $35.8 million award to Northrop on 19 October supports the navy’s decision earlier this year to convert the MQ-25 Stingray into a refuelling asset.

Northrop was one of four companies, including Boeing, General Atomics Aeronautical Systems and Lockheed Martin, that completed preliminary designs for the MQ-25 when it was known as the unmanned carrier-launched surveillance and strike (UCLASS) programme. This was a very different aircraft, designed to penetrate defended airspace, collect intelligence and attack targets.

The navy converted UCLASS into the carrier-based air refuelling system (CBARS) contract earlier this year. Instead of prowling deep behind enemy lines, the CBARS would mainly fly circles around the carrier battle group or escort strike packages, topping off the fuel tanks of manned fighters before they entered defended airspace without the MQ-25.

As a result, the navy has asked the four companies to modify the preliminary designs to reflect the new mission for the aircraft, which is now referred to as the unmanned carrier aviation air system (UCAAS).

Northrop’s contract value fell about $8 million short of the $43 million deals awarded to the other companies, but it was not immediately clear why Northrop received a lesser amount.

Northrop designed and flew its X-47B under the navy’s unmanned combat air system-demonstrator programme, a $1.5 billion effort that proved the feasibility of operating a tailless and unmanned aircraft on a carrier deck. The X-47B also completed the first autonomous aerial refuelling in-flight, although it did so as a receiver aircraft rather than a tanker.