On August 25, 1950, Convair issued a formal proposal for an all-jet swept-winged version of the B-36, initially designated XB-36G. The Air Force was sufficiently interested that on March 15, 1951 the USAF authorized Convair to convert two B-36Fs (49-2676 and 49-2684) as B-36Gs. Since the aircraft was so radically different from the existing B-36, the designation was soon changed to YB-60.
In the interest of economy, as many components as possible of the existing B-36F were used to build the YB-60. The fuselage from aft of the cabin to near the end of the tail remained essentially the same as that of the B-36F. However, the nose was lengthened to accommodate more equipment, and was tapered to a needle-like instrument probe. The conversion to a swept wing had moved the center of gravity farther aft, which necessitated the addition of a retractable tail wheel underneath the rear fuselage. The plan was to leave the tail wheel still extended during the takeoff run, retracting it just prior to rotation. During landing, the tail wheel remained retracted until both the main and nose gears were firmly on the ground. Because of the higher landings speeds that were inherent with a swept-wing design, the design team included provisions for a drag chute in the tail cone, although it is unclear if it was actually fitted to either prototype. The fuselage was a bit longer than that of the B-36F, having a length of 171 feet.
The most readily-noticeable difference between the YB-60 and the B-36F was the swept wing. A wing sweep of 37 degrees was accomplished by inserting a wedge-shaped structure at the extremity of the center portion of the center wing. A cuff was added to the leading edge of the center wing to continue to sweep line to the fuselage. The net result was an increase of wing area to 5239 square feet. The wing span was 206 feet, about 24 feet less than that of the B-36F. The aircraft was also fitted with a new swept vertical tail and a set of swept horizontal elevators. The new swept vertical tail made the YB-60 somewhat taller than the B-36F, the tip of the new swept vertical fin reaching 60 feet 6 inches from the ground.
The YB-60 was to be powered by eight 8700 lb.s.t. J57-P-3 turbojets, housed in pairs on four pods that were suspended below and forward of the wing leading edge, similar to the B-52, but turboprop engines were still considered as a possible option if the jet engines did not work out.
The YB-60 also differed from the B-36F in its crew allocation and in its armament fit. The original YB-60 concept had only five crew members-pilot, copilot, navigator, bombardier/radio operator and radio operator/tail gunner. All were seated in the pressurized and heated forward compartment. All of the defensive armament of the B-36F was omitted, save the twin 20-mm tail cannon that were remotedly directed by the radio operator/tail gunner seated in the forward fuselage via an AN/APG-32 radar in the extreme tail. The K-3A bombing/navigation system, with Y-3A optical and radar bombing sight was retained. The maximum bombload capacity was the same as that of the B-36F, namely 72,000 pounds.
The second YB-60 and any production aircraft were to have the crew increased to nine. Early in the design process, the Air Force asked Convair to add back some of the retractable turrets that had been omitted from the initial design. The upper forward and lower aft turrets wer to be identical to those of the standard B-36F, but the upper aft turret was still to be omitted.
The conversion of 49-2676 to YB-60 configuration began in the spring of 1951. The work was completed in only 8 months, since almost 72 percent of the parts of the YB-60 were common with those of the B-36F. However, the project was delayed by the late delivery of the J57 turbojets, which did not arrive at Convair until April of 1952. The aircraft was rolled out on April 6, 1952. It was the largest jet aircraft in the world at the time.
The first flight of YB-60 49-2676 took place on April 18, 1952, with Convair chief test pilot Beryl A. Erikson at the controls. The Boeing YB-52 took to the air for the first time only three days later. Although there was never any formal competition between the YB-60 and the B-52, the B-52 quickly exhibited a clear superiority. Although the YB-60 had a clear cost advantage over the B-52 (the YB-60 had a 72 percent parts commonality with the B-36 and used much already-proven equipment), the B-52 clearly had a superior performance. The top speed of the YB-60 was only 508 mph at 39,250 feet, more than 100 mph slower than the B-52. In addition, flight tests of the YB-60 turned up a number of deficiencies--engine surge, control system buffeting, rudder flutter, and electrical engine-control system problems. The stability was rather poor because of the high aerodynamic forces acting on the control surfaces acting in concert with fairly low aileron effectiveness. Consequently, the Air Force concluded that there was no future for the YB-60 and canceled the flight testing program on January 20, 1953. At that time, 66 hours of flight time had been accumulated.
The second prototype was never flown at all. Although it was 95 percent complete, it was never provided with any engines and was not fitted with any government-supplied equipment.
After flight test cancellation, Convair vainly attempted to convince the Air Force to continue interest in the YB-60. Convair even offered to complete the remaining B-36s on the production line as B-60s without charging the Air Force any more money. This proposal was turned down. Convair then tried to convince the Air Force that the YB-60 could be used as an experimental test bed for turboprop engines. This proposal was also rejected. Convair even considered trying to adapt the YB-60 as a commercial jet airliner. Nothing came of this idea either. There was even some consideration of using the YB-60 as a test vehicle for the proposed nuclear-powered X-6. This idea went nowhere as well.
Although the Air Force formally accepted both YB-60s in mid 1954, flight testing was already over and the two aircraft had been permanently grounded. The two YB-60s were shunted off to the side of the runway at Fort Worth, where they sat out in the weather for several months. By the end of July 1954, they had both been scrapped, with some of the components that were common with the B-36F being scavenged for spare parts.
Engines: Eight 8700 lb.s.t. Pratt & Whitney J57-P-3 turbojets.
Performance: Maximum speed 508 mph at 39,250 feet. Combat ceiling 44,650 feet. Maximum range 8000 miles. Combat radius 2920 miles with 10,000 pound bomb load. Initial climb rate 1570 feet per minute. An altitude of 30,000 feet could be attained in 28.3 minutes. Ground run 6710 feet, takeoff to clear a 50 feet obstacle 8131 feet. Normal cruising altitude 37,000 feet. Maximum cruising altitude 53,300 feet.
Dimensions: wingspan 206 feet 0 inches, length 171 feet 0 inches, height 60 feet 6 inches, wing area 5239 square feet
Weights: 153,016 pounds empty, 300,000 pounds gross
Armament: Two 20-mm cannon in the extreme tail. Maximum bombload 72,000 pounds.