First flight of the Republic XF-12 Rainbow occurred on February 4th, 1946. Students of history will note the end of World War 2 in August of 1945 so the XF-12 could never have been considered for war time use unless the conflict proceeded another year or two. This undoubtedly removed much of the interest and fervor surrounding the developments of the XF-11 and XF-12 together. Regardless, performance of the XF-12 showcased promising results.
The XF-12 incorporated several design indicatives of note. Despite use of radial piston engines (in which the area around the propeller base is left open to facilitate airflow for cooling), the XF-12 made use of streamlined nacelles with sliding cowl facilities. This maintained a high degree of aerodynamics particularly across the surfaces of the wings as the nacelles ran from ahead of the leading edges to aft of the trailing edges. Air for cooling was, instead, drawn through the wing leading edge sections found between each engine pairing and exhausted at the rear of each engine nacelle. Engines were also turbocharged for increased output at altitude. The wings themselves were straight in their basic design though slightly angled upwards. The fuselage of the XF-12 was as streamlined as possible - from nose to tail - to, again, facilitate airflow to a maximum degree. The undercarriage was fully-retractable and of the tricycle variety - this arrangement now becoming the norm with new-breed aircraft of the time. The nose of the aircraft was further covered in large areas of glass for maximum viewing outside of the aircraft. The large internal space of the aircraft could be reserved for both specialized equipment and extra fuel. The internal makeup of the XF-12 allowed for all-weather operation - which would have given American warplanners a certain tactical edge in-the-field, especially against the Soviets.
Unfortunately for both the Hughes and Republic designs, the end of the war (and the dawn of the jet age) signified the end of the project, forcing the Republic design endeavor into post-war obscurity. The XF-12 was a promising design by any measure - despite the loss of the second prototype - and was slated to be one of the fastest operational four-engine aircraft of its time. In the end, the XF-12 project was outright cancelled as the USAAF (now the USAF, apart from the Army) looked to cut back on war-time spending. It eventually saw little need for a costly and completely new dedicated observation/reconnaissance system in the post-war world. Despite the USAAF ordering 98 of the Hughes XF-11, only two prototypes existed in the end. Similarly, only a pair of XF-12 Rainbow prototypes ("S/N 44-91002" and "S/N 44-91003") were ever completed by Republic and the second was lost over the Gulf of Mexico along with two of her crew after an engine exploded in-flight. The first prototype was damaged in a July 10th, 1947 crash landing forced by a the loss of the right main landing gear leg. This was repaired and the aircraft continued development until the aircraft was officially retired in June of 1952. This prototype was then, rather sadly, used as a target at the Aberdeen Proving Grounds.
The XF-12 also went under the lesser-known "XR-12" reconnaissance designation of the newly-founded US Air Force. Republic was planning on building an elongated passenger airliner version of its impressive XF-12 as the "RC-2" - of course this was never fulfilled. Both American Airlines and Pan-Am held initial interest in the RC-2's design but settled on existing aircraft types once the XF-12 project fell to naught with the US military. Likewise, the US military settled for existing airframes that could roughly accomplish what the XF-11/XF-12 project set out to do - these included the multi-engined Northrop P-61 Black Widow, Lockheed Constellation and the Boeing Superfortress lines.
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