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The Republic XR-12 Rainbow Reconnaissance Aircraft

The 'might have been' Rainbow was beautiful, fast, and finished before it began production

In a long career testing everything from the P-47 Thunderbolt to the F-105 Thunderchief, celebrated test pilot Lin Hendrix regretted few experiences more than parachuting from the sleek, silvery Republic XR-12 Rainbow and abandoning the aircraft to the inexorable force of gravity.

“It was so beautiful and so promising,” Hendrix said in an interview in the 1980s. “It was such a pity to lose it.”

From some angles the XR-12 (initially, the XF-12) looked like a bullet in motion, even when it was sitting still. It may have been the most eye-pleasing propeller aircraft ever built. To some minds, it was not just a U.S. Army Air Forces (AAF) reconnaissance aircraft but also a potential airliner for a possibly lucrative post-World War II aviation market.

Instead, the XR-12 ended up being a “might have been,” its promise never fulfilled. In later years, single-mission reconnaissance aircraft ranging from the SR-71 Blackbird to the RQ-1A Predator demonstrated that Hendrix, test pilot Lowry Brabham and designer Alexander Kartveli were far from wrong: They were simply ahead of their time.

Republic XR-12

The XR-12 design incorporated four massive 3,500-horsepower, 28-cylinder Pratt & Whitney R-4360-37 Wasp Major engines, the epitome of piston engine technology.  Photo courtesy of the Robert F. Dorr Collection

The XR-12 (designated XF-12 prior to June 5, 1948) was built to meet a 1943 AAF requirement that reflected the infatuation of Col. Elliott Roosevelt with the “convoy fighter” – a flying battleship, bristling with guns, that would accompany Allied bombers to Berlin and Tokyo. When it became clear that the escort mission could be handled by the Merlin engine-powered P-51 Mustang, President Franklin D. Roosevelt’s son helped change the purpose of the plane he wanted to long-range reconnaissance.

Fortunately, a requirement for such an aircraft was readily at hand. The AAF had determined that it needed a high-speed, long-range reconnaissance aircraft for operations in the Pacific (it did not yet have experience with the F-13 Superfortress, the reconnaissance version of the B-29 and the next plane in the “F” for “foto” series).  The AAF’s Air Technical Services Command at Wright Field, Ohio, put out what today would be called a request for proposals.


Reconnaissance Rainbow

To complete with an aircraft being developed by Howard Hughes, Republic’s master designer Kartveli sought to attain the prescribed speed and altitude – the so-called “four-four” goal of 400 miles per hour and 40,000 feet – by streamlining the colossal 3,500-horsepower, 28-cylinder Pratt & Whitney R-4360-37 Wasp Major engine, versions of which were used on the B-36 Peacemaker and B-50 Superfortress bombers. Kartveli’s team positioned the engines’ air-cooling inlets in the wing leading edge rather than in the nacelles. The engines incorporated high-speed blowers. An exhaust-driven General Electric turbo-supercharger was located in the rear of each nacelle, gathering thrust of up to one-third of the original horsepower. Each engine drove a 16-foot, four-bladed Curtiss propeller with a bullet-like pointed hub. Other aerodynamic tweaks, plus a carefully designed mid-wing configuration, produced an aircraft with remarkably little drag. In March 1944, the AAF ordered two prototypes (serials 44-91002/91003), but the war was already won when the first of these rolled out at Farmingdale, Long Island, in December 1945.

Republic XR-12

The frontal view of the Republic XR-12 provides a good view of the mid-wing design that lowered drag. The aircraft set a new cross country record by averaging 426 mph between New York and Wright Field, Ohio. Photo courtesy of the Robert F. Dorr Collection

The first ship completed its maiden flight on Feb. 7, 1946 with Brabham at the controls. A proof-of-concept airframe, it carried no mission cameras. The second aircraft, which flew on Aug. 12, 1947 with Hendrix as pilot, was a true reconnaissance aircraft, capable of taking photographs and of furnishing all required film-processing and interpretive facilities in flight, a capability never matched by any other platform during the decades when cameras used film. One proposed configuration had eleven crewmembers with spacious work facilities in the fuselage behind the single pilot.

In May 1946, the Rainbow set a new cross-country speed record for four-engined aircraft, covering 576 miles between New York and Wright Field at 426 miles per hour. A Lockheed Constellation had held the previous record of 375 miles per hour.

On Nov. 4, 1948, this mission-equipped second ship experienced an explosion in its number two engine nacelle, which gave Hendrix and the others in his three-man crew no choice but to hit the silk. Flight testing of ship number one was suspended.


Altered Expectations

Earlier, with a postwar boom in air travel envisioned by many, Republic negotiated with American Airlines for the purchase of 20 RC-2 Rainbow civilian variants for use on high-speed passenger runs. In 1946, Pan American World Airways issued a letter of intent to purchase six RC-2s with options on a dozen more.

The XR-12 had a wingspan of 129 feet 2 inches. The pointy fuselage was 99 feet 8-1/4 inches long. Empty weight was 66,980 pounds and when fully grossed the Rainbow weighed 113,250 pounds. It was bigger and roomier than it looked. Republic envisioned the RC-2 airliner as carrying 46 passengers up to 4,100 miles at a cruising speed of 410 miles per hour. The military XR-12 had a maximum speed of 460 miles per hour at 28,000 feet, making it comparable in speed with the final propeller-driven fighters in inventory, planes like the F-51H Mustang and F8F-2 Bearcat.

Republic XR-12

The Republic XR-12 has been described as the most beautiful propeller plane ever built. Photo courtesy of the Robert F. Dorr Collection

Even when tests of the competing Hughes aircraft were ended – the XR-11 (earlier, the XF-11) had also been built in two prototypes and one had crashed, almost killing pilot Hughes – the Air Force (which emerged from the AAF to become an independent service branch on Sept. 18, 1947) in 1948 stood by the XR-12. The airlines were first to waver, perhaps because new-built, post-war Douglas DC-4s were coming off the production line, with DC-6s soon to follow.

The loss of the working second prototype ended the Air Force’s support and was the death knell for the “might have been” XR-12 Rainbow. Several years before President Dwight D. Eisenhower’s proposal for an “Open Skies” exchange with the Soviet Union, the project was shelved. At one point, an order had existed for 20 R-12As, but was diverted to RB-50 Superfortresses. When the project was canceled, a further production order for six R-12A Rainbows was canceled, too.

The prototype in all of its elegant glory was placed into storage. And then, this beautiful and promising aircraft was carted off to Aberdeen Proving Ground, Md., where it was used as an artillery target and destroyed.


Robert F. Dorr is an author, U.S. Air Force veteran, and retired American diplomat who...