The X-47B aircraft built by Northrop Grumman recently passed another milestone: the first autonomous, air-to-air refueling by an uncrewed air vehicle (UAV). Or at least it’s the first one that’s been made public.

Air-to-air refueling is a critical part of modern combat operations, so much so that virtually every new military aircraft is equipped to receive fuel from a guided boom or unguided hose-and-drogue. Large militaries—it takes a lot of money to fly tankers—practice the technique routinely. Though it takes precision flying to tank in any circumstance, the hose-and-drogue method means the pilot (or in this case UAV) must catch the dangling fuel line itself.

The demo was a long time coming. In 2012 two enormous NASA RQ-4 Global Hawks connected in mid-air, though no fuel was passed. Two crewed aircraft, an F/A-18 and a contractor-flown Learjet, used the X-47B’s software to approach and/or hook in to a tanker while the pilots watched, but no fuel was actually passed.

Perhaps the most interesting thing about the April 22 test is what’s not being said. The X-47B’s predecessor, the kite-like X-47A Pegasus, came out of a Pentagon requirement to build an uncrewed combat air vehicle (UCAV) demonstrator. The results of that competition were Pegasus and Boeing’s X-45, both now retired (one X-45 is exhibited at the National Air and Space Museum).

The Navy went on to commission the X-47B to make further demonstrations, but a close observer might note that there seems to be a step or two missing in this research program. Flight test is a cautious, incremental business, and hose-and-drogue refueling is more difficult for any aircraft than the Air Force’s boom tanking method. What happened to the boom tanking demonstration?

The missing link, as it were, would likely be a stealthy, land-based UCAV with boom refueling capabilities. Since no aircraft fitting that description have been made public, the answer to the mystery is probably in the murky world of secret airplanes flown from remote bases in Nevada and California. One such aircraft, Lockheed’s RQ-170, was officially declassified only after one of them very publicly crashed in Iran in 2011. The RQ-170 was long rumored to have flown a “pre-contact” profile with a boom tanker several years ago, a rumor recently given additional credence after alleged pictures of the flight showed up online. The RQ-170 may or may not have refueling capabilities, but certainly would not approach a tanker as closely as pictures suggest without good reason. If not mid-air refuelable (and no pictures clearly suggest that capability), there would likely either be another version of the aircraft, or it would be used as a testbed to reduce risk for follow-on aircraft.

It’s long been known, based on public statements, that Northrop has a large secret UAV program. A 2013 article in the trade magazine Aviation Week & Space Technology speculated about an aircraft it called the RQ-180, but gave little detail beyond its provenance (X-47B-like) and relative size (significantly larger than X-47B).

In any case, what the Navy learned from X-47B tests will feed into the Unmanned Carrier-Launched Airborne Surveillance and Strike (UCLASS) program, meant to produce the first big, fixed-wing UAV to fly from an aircraft carrier. Though the Navy was at first looking for a relatively small aircraft, mainly for stealthy reconnaissance but with a payload/armament bay built in—effectively a mass-produced X-47B—the requirements have kept shifting. While some in the Pentagon favor an X-47B-like aircraft, another group is pushing for a heavier, more combat-oriented airplane. Yet another, smaller group presses for a less capable aircraft, that could replicate the un-stealthy capabilities of the Air Force’s RQ-9 Reaper.

So what happens to the X-47Bs—two of them were built—now that testing has finished? Northrop, though cagy when discussing ongoing programs, is clear that its entry in the UCLASS competition will be sufficiently different that simply modifying the X-47 isn’t practical. UCLASS competitor Lockheed and several politicians have recommended continued testing of the X-47B, which would provide information that could help UCLASS designs. Though such aircraft are often reused for other test programs, the Navy once appeared intent on sending them to museums for display.

The X-47B’s air-to-air tanking demonstration is one more step to maturity for UAVs, and an impressive achievement. But to write the complete history of how this technology is advancing, one would have to look to California and Nevada, where there are—for now—more questions than answers.