In the late 1950s, the USAF was all about speed and altitude. The Mach 2 capable B-58 Hustler started flying in 1956 and entered service in 1960. The Mach 2.3 capable F-106 Delta Dart entered service in 1959. That wasn't enough. Planners wanted more. They wanted planes that would go farther than the B-58 and much, much faster.
The XB-70 takes to the skies
The Mach 3+ XB-70 Valkyrie was under development and promised to push the state of the art beyond anything that was flying in the world. Mach 3 requires thrust levels well beyond what it takes to go Mach 2 and it takes a lot of fuel to make that power. That means a large aircraft. To keep weight down, and to deal with the high skin temperatures the plane would see, exotic materials were being developed such as steel brazed honeycomb panels and new ways were being developed to work with titanium.
The XB-70 and XF-108's ejection capsule being tested
The mindset of the times in the US dictated that if we were working on a technology then so were the Soviets. That meant we needed a high speed interceptor with capabilities similar to the XB-70. We wanted to intercept high speed Soviet bombers as far from North American targets as possible. That meant range and power. The natural solution was to piggyback off of XB-70 technology and develop an interceptor with similar performance characteristics.
Here a YB-58 tests the J-39 engine used on the XB-70 and proposed for the XF-108
The XF-108 Rapier program was started in 1955 by North American Aviation, who was also building the XB-70, as an answer to the Air Force's desire to obtain a very high speed interceptor and bomber escort. The aircraft utilized a number of design features that were common to the Valkyrie. The XF-108 would use the same 29,000 pound thrust afterburning GE J-39-GE-3 engines as the XB-70 as well as the escape pods and many of the advanced materials. A number of the aerodynamic tricks employed on the XB-70 would also see their way into the Rapier's design. These included drooped wing tips to take advantage of compression lift and a number of design feature that were intended to improve directional stability.
While North American was working on design refinements a new variable entered into the equation. The Soviets were able to show that they had the technology to deploy nuclear weapons via a ballistic missile. This meant that there would be little need to deploy a large fleet of bombers against the US to achieve their goals. Missiles could do the same job, for less cost and with little to no hope of interception. In addition, the Soviets paranoia about the development of the XB-70, and other fast and high flying aircraft, led them to develop significant ground to air missile capability. As the 1960s approached funding for the XB-70 was cut by Presidents Eisenhower and Kennedy to allow for two airframes and some research flying and the XF-108 was canceled all together.
The XF-108 never made it to the point where an aircraft was built. All we have now, at least what is available on the open internet, are pictures of the full scale mockup and artists renditions. If it would have flown it would surely have won the hearts of aviation enthusiasts in the same way the XB-70 and SR-71 did. It was a good looking plane and everyone loves the speed demons..
Images via GIS: kavok.com.br, free-images.gatag.net, Wikipedia.org, milaviate.com, survivecity.com
Information: National Museum of the United States Air Force Fact Sheet, Wikipedia.org, milaviate.com