Somewhere in the Nevada desert, a Boeing 737 sits in a hangar that does not officially exist. It looks almost normal from a distance — the familiar cigar fuselage, the low-slung JT8D engines, the T-tail absent because this is a 737, not a DC-9. But look closer and things get strange. A large dorsal turret bulges from the spine. Sensor enclosures of unknown purpose sprout from the fuselage. And on its transponder, when it bothers to squawk at all, it broadcasts the callsign RAT55. This is the NT-43A, serial number 73-1155 — the most secretive Boeing 737 ever built and one of the least understood aircraft in the US Air Force inventory. Its latest appearance over Florida during the Artemis II launch in April 2026 has revived questions about an aircraft that the Pentagon would very much prefer nobody asked about.

From Navigation Trainer to Black Project

The story begins in the early 1970s, when the Air Force bought 19 Boeing 737-200s under the T-43A designation. Their job was simple: train navigators. The aircraft carried banks of navigation stations where students practiced celestial, radar, and inertial navigation techniques. It was the most boring mission a 737 ever flew.

One of those trainers, tail number 73-1155, took a very different path. At some point — the exact date remains classified — the aircraft was pulled from the training fleet and handed to Denmar Technical Services, a company that specialises in extremely high-end radar and signature testing for the Pentagon. What emerged was the NT-43A: a flying radar laboratory equipped to measure the radar cross-section of stealth aircraft in flight.

What RAT55 Actually Does

The “N” prefix in military designation language means “permanent special test.” The NT-43A is believed to be the final validation platform for stealth technology — the last stop before a new low-observable aircraft, coating, or structural modification receives its operational blessing. When Lockheed Martin finishes a new batch of radar-absorbent material for the F-35 or B-21, the NT-43A is likely the aircraft that flies alongside the test subject, illuminating it with radar and measuring exactly how invisible it really is. The dorsal turret that gives the aircraft its distinctive profile houses a radar array that can rotate and track targets at various angles and frequencies. Plane spotters who have photographed RAT55 over the Mojave Desert describe additional sensor enclosures along the fuselage — each presumably covering a different portion of the electromagnetic spectrum.

The Artemis II Surprise

On April 1, 2026, the NT-43A appeared over Florida, tracked by civilian flight-monitoring services under its familiar RAT55 callsign. The timing was unmistakable: NASA’s Artemis II mission — the first crewed lunar flight in over half a century — was launching from Kennedy Space Center that same day. Why would a stealth-testing radar aircraft monitor a space launch? The most plausible explanation involves missile tracking and space surveillance. The NT-43A’s radar suite, designed to track small, fast-moving objects with minimal radar signatures, is exactly the kind of sensor you would want pointed at a rocket ascending through the atmosphere. The Space Force has been expanding its tracking capabilities, and repurposing a proven testbed for launch monitoring fits the pattern.

Hiding in Plain Sight

The NT-43A is a paradox. It is an acknowledged aircraft — it appears in Air Force records, it squawks a transponder code, and it has been photographed repeatedly. Yet almost nothing about its specific capabilities, operational history, or current mission set is publicly documented. It operates from Tonopah Test Range Airport, the same facility that once hid the F-117 Nighthawk fleet from the world. It flies patterns over restricted airspace that would make a UFO investigator’s head spin. For a 50-year-old Boeing 737, the NT-43A has lived one of the most extraordinary lives in aviation. It trained navigators, became a flying radar laboratory, helped validate the stealth technology that defines American air power, and now watches rocket launches from the edge of space. Not bad for an airframe that started life as the most mundane airliner ever built. Sources: The War Zone, FlightGlobal, Army Recognition, The Aviationist