The U.S. Navy has awarded four manufacturers contract amendments with additional funding to refine their concepts for the MQ-25 Stingray refueling drone. The service’s requirements for the aircraft now emphasize the tanking role over multi-mission capability, according to one of the manufacturers.

On April 13, the Department of Defense (DOD) announced contract modification awards to Northrop Grumman ($24.7 million), Boeing ($19.1 million), Lockheed Martin ($18.8 million) and General Atomics ($18.7 million) to refine their air vehicle concepts and develop technology trade-off alternatives in advance of the MQ-25 engineering and manufacturing development phase. The DOD expects work to be completed on the contract add-ons in September 2018.

The four companies each received previous, risk-reduction contracts of roughly double the latest amounts in October. Since at least 2011, they have participated in studies and preliminary design exercises for what the Navy originally called the unmanned carrier-launched airborne surveillance and strike (UClass) program.

Over time, the Navy’s expected requirements for the aircraft have evolved from it being a multi-mission platform capable of performing intelligence, surveillance, reconnaissance (ISR) and strike missions to one focused on carrier-based aerial refueling, a role the F/A-18E/F Super Hornet now serves. Competing manufacturers expect the service will issue a formal request for proposals this summer.

“It’s a tanker, and that’s pretty well what the Navy has defined as the primary requirement for this unmanned system,” Rob Weiss, general manager of Lockheed Martin’s Skunk Works advanced development unit, told reporters March 21 during the company’s annual media day. “They’re frankly doing all the right things to accelerate this program and get it in the hands of the warfighter sooner than later. The focus is on mission tanking; we’ve done a number of ([technology)] trade-offs on the actual configuration.”

During the same event in 2016, Weiss said Lockheed Martin saw no reason to offer a traditional wing-body-tail design if the plan wais to eventually operate the aircraft in contest airspace. Lockheed Martin was earlier thought to be developing a stealthy, flying-wing aircraft along the lines of its RQ-170 Sentinel, with Northrop Grumman and Boeing proposing advancements of the X-47B and Phantom Ray flying wings, respectively. General Atomics has promoted the Sea Avenger, a marinized version of its jet-powered Predator C with twin, canted tails.

At the media day this year, Weiss displayed a graphic of an underwing pod or tank trailing a hose and refueling drogue to a following F/A-18, hinting at Lockheed Martin’s latest air vehicle configuration. “If ([the Navy’s)] requirements were about penetrating ISR, where it’s in contested airspace—be it ISR or strike—you would need an airplane that looks different than a concept with pylons,” he said. “But their requirements have now been defined to be a tanker, so you really don’t want to go with a tailless design.”