A TOP-secret, one-of-a-kind military plane dubbed “The Rat” has been spotted completing a shadowy mission – but nobody knows what it’s up to.

Rat55 is a highly-modified Boeing 737 with a bizarre bulbous nose ad and an enlarged tail.

It is dedicated to covert missions, with speculation it collects radar data on other stealth aircraft.

Aviation enthusiasts caught The Rat on one of its very rare outings this week – and even managed to snap some pics.

The plane flew across the country and made two stops – one in Arkansas and one in Texas.

It took off from the Nevada Test and Training Range and travelled to Northwest Akansas National Airport, stopping in Texas on the way back.

Rat55 touched down at Rick Husband International Airport in Amarillo, Texas, “at 2:40 pm and departed at 3:40 pm.

“It was cleared back to Tenopah Test Range Airport,” according to plane photographer Jason Zicker.

Jason said a fellow plane spotter had alerted him to the exciting guest appearance.

He said: “As far as I know, it was just for gas.

“RAT55 did have a flight plan to come here a couple of months ago, but never showed up.”

The mission is a total mystery.

Air Force Materiel Command refused to provide any details in response to a request by TWZ.

One theory is that the trip could have been linked to a company called King Aerospace, which specializes in heavy maintenance and deep modification.

The Rat visited a King Aerospace site in Oklahoma in 2022, and the company also works from the Northwest Arkansas National Airport.

The name RAT stands for Radar Airborne Testbed – but also suits the aircraft down to a tee.

The weird modifications – heavy on the nose and tail – make the plane look decidedly rodent-like.

Rat55 seems to live at the high-security Tonopah Test Range Airport (TTR), famous for its shadowy aircraft programs.

It also spends time in the skies near Area 51, reports TWZ.

In the past, its trips were orchestrated to avoid public sightings – but that seems to no longer seems be the case.