Convair B - 36 variants Convair B - 36 variants began in 1941 with the XB - 36 to meet the strategic needs of the US Army Air Forces ; then later becoming United States Air Force with its Strategic Air Command . In 1948 , the B - 36 immediately becoming a mainstay of the American nuclear deterrent . It underwent a number of design changes before being withdrawn from service in 1959 . It was also well suited to high altitude very long range reconnaissance missions , and several alterations were made with this mission profile in mind . XB - 36 . In 1941 , the fall of Britain to a German invasion seemed imminent . If the United States joined the war against the Axis Powers , the fall of Britain would leave no bases in Europe from which the United States Army Air Corps ( AAC ) could bomb Germany . This possibility led to AAC to seek a bomber of truly intercontinental range . On 1941 April 11 , the AAC issued a design competition for an aircraft with a 275 mph ( 445 km / h ) cruising speed , a service ceiling of 45 , 000 ft ( 14 , 000 m ) , capable of delivering 10 , 000 lb ( 4 , 500 kg ) of bombs to targets 5 , 000 miles ( 8 , 000 km ) away . At the time , these requirements far exceeded the best technology available . On 1941 November , the United States Army Air Forces signed a contract for two experimental aircraft under the designation XB - 36 , based on design studies previously submitted by Consolidated Aircraft Corporation . A few days later , the Engineering Division at Wright Field decided that Consolidated ' s six - engine design , with all engines on the trailing edge of the wing , was the best option for the aircraft . The original design used vertical stabilizers and rudders , similar to those used in the B - 24 Liberator . By the time the first XB - 36 was delivered , the design had been changed to a huge single tail , and the wingspan increased to 230 ft ( 70 m ) , an unprecedented size . It was clear that the B - 36 needed nose guns . Adding these required a substantial rearrangement of the cockpit . In late 1944 , a mockup of the new nose was approved . Though too late for the first prototype , it would be fitted to the second XB - 36 . Unfortunately , the new nose with its radar and radio equipment added significant weight . The first ( and only ) XB - 36 was completed at the Consolidated Fort Worth factory in late 1945 . The plane sat on huge wheels 9 ft ( 2 . 8 m ) in diameter ; only three airfields in the USA had concrete thick enough to support the pressure exerted by wheels of that size . In 1948 June , the single - wheel undercarriage was replaced by a new undercarriage consisting of two wheels with half the diameter on each strut . This design , which would became the production standard , enabled the B - 36 to operate on runways of reasonable thickness . The maiden flight revealed problems with the flap actuating system , engine cooling , and the reliability of the aluminum wiring . Propeller vibration adversely affected the wing structure . Worse yet , the XB - 36 failed to meet the 1941 specifications . In particular , inadequate engine cooling kept its service ceiling under 30 , 000 ft ( 9 , 000 m ) , much less than that called for in the original contract . Following a grounding for modifications , the XB - 36 was flight tested for 160 hours by pilots of the Army Air Force Air Materiel Command , then returned to Convair for further testing . References : YB - 36 . The YB - 36A , a conversion of the original XB - 36 , was turned over to the United States Air Force in June 1948 , a week before the production B - 36A was scheduled to be delivered . The YB - 36A had inadequate operation value , and the Strategic Air Command used it primarily for training missions . Later it was idle for several years , then employed in the Carswell Air Force Base firefighting program . The second prototype , designated YB - 36 , of which only a single instance was built , was chosen as the production version in mid - 1945 . Its new high - visibility cockpit and redesigned nose compartment became the standard . Thanks to various design improvements , the YB - 36 outperformed the XB - 36 , eventually reaching altitudes of 40 , 000 ft ( 12 , 000 m ) . It was essentially built to B - 36A specifications ; in fact , it flew six months " after " the B - 36A . In late 1950 , the YB - 36 was returned to Convair and converted into a reconnaissance aircraft under the designation RB - 36E . In the spring of 1957 , it was decommissioned and placed in the National Museum of the United States Air Force at Wright - Patterson Air Force Base . References : B - 36A . The B - 36A included several of the new elements developed on the YB - 36 , including the domed canopy and the four - wheel main landing gear ( as opposed to the single - wheel landing gear used on the XB - 36 and the YB - 36 ) . These new features were in a sense first seen on the B - 36A rather than the YB - 36 , because it was the former that flew first — by six months . The first B - 36A ever built was destroyed by a series of brutal stress tests , consisting mainly of applying more and more load to various parts of the aircraft until they disintegrated . On April 8 , 1948 , a B - 36A made a 6 , 922 ( 11 , 140 km ) flight of 33 hours , shuttling between Fort Worth and San Diego three times without landing , and carrying a 10 , 000 lb ( 4 , 500 kg ) bomb load . In May , the same aircraft did a round trip of 8 , 062 miles ( 12 , 970 km ) lasting 33 hours . By virtue of its designation , the B - 36A was technically a production aircraft . However , it lacked defensive armament because the system wasn ' t ready . It was an explicitly interim plane used by the 7th Bomb Group at Carswell Air Force Base in Texas for the sole purpose of flight testing and transitional crew training . When the newer B - 36B became available , all B - 36As were converted to reconnaissance aircraft and redesignated RB - 36E . A total of 22 were built . References :