North American F-107 Ultra Sabre: The All-Weather Fighter-Bomber That Was Cancelled

The North American F-107 Ultra Sabre was one of the most distinctive American aircraft of the 1950s. Developed as an all-weather fighter-bomber and originally designated the F-100B, it was intended to take the F-100 Super Sabre into a faster, more capable generation. Its unusual dorsal air intake, automatic variable-area inlet duct, all-moving vertical fin, and provision for a semi-submerged weapon made it one of North American Aviation's most ambitious designs.

The F-107 was fast and technically sophisticated, but it was also difficult to develop. Its flight-test program exposed serious inlet and handling problems, while its competitor, the Republic F-105 Thunderchief, was judged closer to production. Only three F-107A prototypes were built, and the United States Air Force cancelled the program in 1957. Even so, the Ultra Sabre remained valuable as a high-speed research aircraft and an important chapter in the evolution of supersonic aviation.

F-107 Ultra Sabre on Film

These three videos offer a visual introduction to the F-107 Ultra Sabre's unusual design, high-speed mission, and place in North American Aviation history.

North American Aviation F-107 — Video 1

North American Aviation F-107 — Video 2

North American Aviation F-107 — Video 3

From the F-100B to the F-107 Ultra Sabre

The design began as a proposed development of the F-100 Super Sabre under the designation F-100B. It soon became clear that the Air Force requirement for a fully enclosed internal bomb bay and a more advanced all-weather fighter-bomber would require a substantially new aircraft rather than a simple derivative.

North American's answer was a large, single-engine supersonic aircraft with a weapon carried in a semi-submerged recess in the lower fuselage. To create space for this arrangement, the designers placed the jet intake above and behind the cockpit. This gave the F-107 its famous humped-back appearance and left the underside available for weapons carriage, but it also created difficult engineering and pilot-safety questions.

Maiden Flight and the Republic F-105 Competition

The first F-107A made its maiden flight on September 10, 1956. The aircraft entered a competition with the Republic F-105 Thunderchief for the Air Force's all-weather fighter-bomber requirement.

In flight trials, the Ultra Sabre demonstrated Mach 2-class performance and the potential of its advanced aerodynamic layout. However, the F-105 was considered closer to production and offered a more practical path to an operational aircraft. In March 1957, the USAF made its final decision to cancel the F-107 program and proceed with the F-105 Thunderchief instead.

Why the F-107 Was Cancelled

The F-107 program was cancelled for a combination of design trade-offs, changing military priorities, and industrial timing. The decision was not simply a judgement that the aircraft was too slow. Rather, the Air Force had to choose the aircraft that could deliver the required tactical nuclear mission with the least remaining development risk and the fastest route to production.

Semi-Submerged Weapon Versus Enclosed Bomb Bay

The F-107 carried its principal weapon in a semi-submerged belly recess, while the Republic F-105 offered a fully enclosed internal bomb bay. For the Air Force's tactical nuclear mission, the F-105's arrangement was considered more practical. An enclosed bay could reduce aerodynamic drag after the weapon was released, helping the aircraft retain speed while leaving the target area.

Production Readiness and Timing

The F-105 program was further along and closer to mass production. The Air Force wanted to field tactical fighter-bombers quickly, so the aircraft requiring less additional development carried a major advantage. The F-107 order was reduced from a planned production quantity of 33 aircraft to the three prototypes that were ultimately built.

No Final Direct Fly-Off

Although the F-107 and F-105 were competing designs, a definitive direct fly-off between the two aircraft did not take place in the way later aviation stories sometimes suggest. Delays affecting the F-105 prototype meant that the Air Force relied heavily on engineering projections, mission requirements, and existing production timelines when making its selection.

Unique Design Features

Dorsal Jet Intake

The F-107's most recognizable feature was its overhead air intake, positioned above and behind the cockpit. This arrangement freed the lower fuselage for a semi-submerged weapon installation and helped create the aircraft's distinctive profile. It also made the intake system central to the aircraft's performance and flight-test problems.

Automatic Variable-Area Inlet Duct

The aircraft used an automatic variable-area inlet duct intended to regulate airflow to the engine during high-speed flight. Like several Century Series aircraft, the F-107 required extensive work to tune its inlet system at high Mach numbers. When operating correctly, the system supported the aircraft's Mach 2-class performance, but the automatic mechanism became a chronic source of trouble.

All-Moving Vertical Fin

The F-107 incorporated an all-moving vertical fin. Rather than relying only on a hinged rudder, the complete vertical tail could move to provide directional control at high supersonic speeds. This was an advanced feature for the period and reflected North American's effort to give the aircraft reliable control authority throughout its intended performance range.

Spoiler Roll Control

The F-107 used spoilers on the wings instead of conventional ailerons for roll control. Test pilots valued the ease with which the aircraft could be rolled, including at very high speed. This arrangement was another example of North American's willingness to move beyond familiar solutions in pursuit of supersonic performance.

Weapons and All-Weather Mission

The F-107 was designed as an all-weather fighter-bomber, with four 20 mm M-39 cannons and provision for carrying a nuclear weapon in a semi-submerged belly recess. The weapons requirement strongly influenced the aircraft's unusual intake and fuselage arrangement. The design was therefore not simply a faster fighter, but an attempt to combine supersonic performance, radar-directed all-weather operations, and tactical strike capability in one aircraft.

What the F-107 Did Well

Test pilots recognized that the F-107 offered impressive capabilities despite its difficult development. Its strengths included excellent high-speed handling, strong directional control, and an inlet system designed to maintain engine airflow beyond Mach 2. The aircraft's spoilers gave it responsive roll control, while the all-moving vertical fin supported directional control at supersonic speed.

The F-107's experimental value also extended beyond the aircraft itself. The side-stick evaluation and related control research connected the program with the developing X-15 research effort. The aircraft's flight-control and intake work formed part of the broader body of knowledge that informed later high-speed projects, including the X-15 and North American's A-5 Vigilante.

Flight-Test Problems

The F-107's problems were not limited to one component. The automatic variable inlet duct demanded continual attention, particularly during high-speed operation. Inlet tuning at high Mach numbers was difficult, and later evaluation also identified unsuitable flying qualities. These issues did not erase the aircraft's speed potential, but they made the design harder to mature into a dependable operational fighter-bomber.

During testing, the variable-area inlet system became a constant mechanical challenge. Engineers eventually had to lock the inlet into a fixed position for some research work. This simplified the system but reduced performance, limiting the aircraft to approximately Mach 1.2 in that configuration. The difficulties also contributed to a sustained-flight altitude limit of approximately 51,000 feet during the test program.

The F-107's Operational and Safety Disadvantages

Restricted Rearward Visibility

The large dorsal intake and engine hump immediately behind the cockpit severely restricted the pilot's view to the rear. At a time when some planners expected future combat to depend mainly on long-range guided missiles, this seemed an acceptable compromise. In a close visual engagement, however, the lack of rearward visibility would have been a serious tactical disadvantage.

Ejection and Intake Hazards

The position of the engine intake behind the cockpit also created difficult ejection-safety questions. An ejecting pilot had to clear the intake structure and engine area, so the escape system was designed to drive the seat through the canopy with enough force to pass over the aircraft's raised centre section. This unusual arrangement contributed to the F-107's grim reputation as a potential “man-eater” in some accounts.

Unusual Cockpit Entry

The canopy opened by sliding vertically upward rather than by using a conventional hinged arrangement. This gave the aircraft a distinctive cockpit design but added another layer of mechanical complexity to a machine already dependent on several unconventional systems.

The first prototype encountered so many problems that it was eventually grounded. Instead of returning to flight, it became a source of spare parts for the other two aircraft. This left the second and third prototypes to carry most of the remaining test and evaluation work.

NACA Research and the F-107 Side-Stick Evaluation

After the Air Force cancelled the program, the first and third prototypes were leased to NACA, the National Advisory Committee for Aeronautics and predecessor to NASA, for high-speed flight research. The variable intake on the third aircraft was one of the main subjects of investigation.

According to Ted Spitzmiller's account in Century Series: The USAF Quest for Air Supremacy 1950-1960, NACA and the USAF also evaluated a side-stick control installed on the right side of the F-107A cockpit. The research related to the hand-controller planned for the X-15, where supporting the pilot's arm would help reduce unintended control inputs during rocket-powered, high-acceleration flight. The traditional centre stick would remain useful after engine burnout.

The third prototype, F-107A 55-5120, was transferred to NACA in February 1958 and completed approximately forty flights during the research program. Test pilot Scott Crossfield was flying the aircraft during a take-off when control difficulties led to a ground loop. Both main tyres were blown and a small brake fire followed. The aircraft was not considered badly damaged, but it was not repaired. The side-stick evaluation was judged to have demonstrated the desired characteristics, and the airframe was later cut up for use as a fire-fighting training form.

The Three F-107A Prototypes

• 55-5118: The first prototype. After being grounded, it became a source of spares and was later acquired by the Pima Air & Space Museum in Tucson.
• 55-5119: The second prototype, used for weapons testing with conventional and atomic weapons. It is preserved at the National Museum of the United States Air Force in Dayton, Ohio.
• 55-5120: The third prototype, used by NACA for variable-inlet and side-stick research. Following its take-off accident, it was not returned to flight.

Technical Specifications

Why the F-107 Still Matters

The F-107 Ultra Sabre is sometimes remembered simply as the aircraft that lost the competition to the F-105. That description misses the importance of the program. The aircraft tested an unconventional intake arrangement, explored the difficult relationship between weapons carriage and supersonic aerodynamics, and provided NACA with a useful platform for studying high-speed flight controls and inlet behaviour.

Its story also shows why prototype aircraft matter. A design can reach remarkable speed and incorporate genuinely advanced features while still proving too complex, too unreliable, or too late to become an operational aircraft. The lessons from the F-107 contributed to the broader development of supersonic fighters and to the research environment surrounding the X-15.

F-107 Ultra Sabre Technical Documentation

Online Aviation Library is preparing a technical manuals collection for the North American F-107 Ultra Sabre. The collection will support researchers, aviation historians, restorers, and collectors studying this rare prototype fighter-bomber and its flight-test history. A direct product link can be added here once the F-107 manual listing is live.

Sources and Further Reading

Ted Spitzmiller, Century Series: The USAF Quest for Air Supremacy 1950-1960, is an important source for the F-107 flight-test and NACA research history discussed in this article. Museum records and original technical documentation should be consulted for individual airframe details and configuration-specific performance data.