You know what a B-17 looks like. Four radial engines. Twin tail. Glass nose. Heavy-bomber Americana, stamped onto a thousand black-and-white photographs of the daylight raids over Germany. Now picture one with a fifth engine bolted to its nose — a giant turboprop, or sometimes a roaring jet, mounted where the Plexiglas bombardier station used to be.

That happened. Curtiss-Wright Corporation flew exactly this aircraft for nineteen years, from 1947 to 1966, out of a small airport in Caldwell, New Jersey, testing every new piston, turboprop, and jet engine the company built. The bomber was officially designated XB-40 in some references and JB-17G in others, but most of the people who flew it just called it “the testbed.”

Why a B-17?

By 1947 the B-17 was obsolete. The Air Force was retiring them by the hundred and selling them as surplus to anyone with a hangar and a dollar. Curtiss-Wright bought one, took it to its New Jersey factory, and asked: what if we used this as a flying engine test stand?

The B-17 had three things going for it. It was huge — long enough to mount a substantial engine on the nose without overloading the centre of gravity. It was already built to handle the asymmetric thrust of four wing-mounted engines, so a fifth engine in front did not destabilise it. And it was cheap. The whole airframe cost less than a single new engine to acquire.

A Fifth Engine, A New Job

Curtiss-Wright’s engineers stripped out the bombardier station, the chin turret, and most of the forward fuselage equipment. In its place they bolted a structural test mount sized for almost any contemporary engine. The aircraft’s four original radial engines stayed in place, providing reliable propulsion at all times. The fifth engine — the one being tested — could be started, run at full power, instrumented, and shut down in flight without ever putting the aircraft itself at risk.

This mattered. Before the testbed, every new aircraft engine had to fly on a prototype aircraft, and prototype aircraft crashed. The B-17 testbed gave Curtiss-Wright a flying laboratory where engineers could log thousands of hours on a new engine before it ever entered a production aircraft.

Engines That Flew on the Testbed

The Wright J65 turbojet — used in the F-84F Thunderstreak, the F-100A Super Sabre, the B-57 Canberra, and the F-11 Tiger — was qualified on the B-17 testbed before it ever flew on a production fighter. The Wright XT35 turboprop, intended for the giant Convair B-36 Peacemaker successor, ran on the B-17 for hundreds of hours during early Cold War testing. Half a dozen smaller piston experiments — radial engines, opposed-cylinder concepts, even a turbo-compound prototype — all logged their first flying hours on the testbed.

The aircraft was, in effect, a flying spec sheet. Anything Curtiss-Wright wanted to sell to the Pentagon had to first prove itself bolted to the nose of a 25-year-old World War II bomber.

A Quiet Retirement

By 1966, jet engines had reached the point where ground-based test cells could simulate flight conditions accurately enough that an actual flying testbed was no longer necessary. The B-17 was sold for scrap. Curtiss-Wright lost most of its engine business to Pratt & Whitney and General Electric within a decade. The company that had built the engines for the Wright Brothers’ first aircraft and most of the bombers of World War II faded from the aviation industry by the mid-1970s.

The B-17 testbed itself was not preserved. There are perhaps two dozen surviving B-17s in the world today, all painted in their wartime livery, all flown at airshows in their original four-engine configuration. None has the fifth engine. None is on the engineering record as the strangest-looking Flying Fortress ever to operate. Which is a shame — because for nearly two decades, this one aircraft had a more interesting flying career than almost any other B-17 alive.

Sources: Curtiss-Wright Corporation archives, Smithsonian National Air and Space Museum, “B-17 Flying Fortress: The Symbol of Second World War American Air Power” (Holmes), Caldwell-Wright Airport historical records.