In May of 1947, Maj. Gen. Curtis E. LeMay, at that time Deputy Chief of Air Staff for Research and Development, wrote a letter to Lt. Gen. Nathan F. Twining, chief of the Air Materiel Command, to request that work begin on a new jet-powered medium bomber that would be ready for service by the late 1950s. The new bomber should have a combat radius of 2500 miles, a cruising speed of at least 500 mph, and a gross weight of 170,000 pounds. It was proposed that the development of such an aircraft would follow the development of the B-52.
General LeMay's proposal led the Air Staff to solicit ideas from the leading US maker of bombing aircraft, the Boeing Airplane Company, as well as from several other manufacturers. At this stage, the project was still rather ill-defined. By October of 1947, things had begun to firm up sufficiently so that the War Department submitted a requirement for a new medium bomber to the aviation industry. The aircraft was to weigh less than 200,000 pounds, have a 2000 mile radius, and be able to carry a 10,000 pound bombload. The aircraft was tentatively assigned the designation XB-55. Boeing submitted the winning proposal, and a Phase I contract for the XB-55 was initiated with FY 1948 funds.
However, in the immediate postwar environment, funding for any type of military project was in short supply and it was decided that the initial design study for the XB-55 would be converted into a purely paper study to explore new aeronautical technologies. As part of the project, the Air Force began to explore the potential of delta wing configurations and began to consider the possibility of bomber designs capable of supersonic flight. Some of this work had actually gotten started before the advent of the XB-55 project, and several companies had launched informal internally-funded studies.
On January 27, 1949, the AMC was directed to cancel the XB-55, since the projected B-47 production rate had reached the point that another subsonic medium bomber would probably be unnecessary. However, the general requirement for a high-performance medium bomber remained intact.
Among the initial approaches to the design of a long-range supersonic bomber was the Generalized Bomber Study (better known as GEBO), which had been carried out by several aircraft companies, in particular Convair. GEBO began with the exploration of the the feasibility of a delta-winged aircraft weighing about 150,000 pounds. This had begun in October 1946 under an Air Force contract given to Convair.
One of the positive results of the shelving of the XB-55 project was that it freed up some scarce funds for additional development. Brig. Gen. Donald Putt, Director of the Research and Development Office and Deputy Chief of Staff for Materiel, recommended that the AMC ask the aircraft industry for new and possibly unconventional proposals for intercontinental bombers. A second Generalized Bomber Study (known as GEBO II) was initiated. The design parameters were a radius of 1200 to 2500 miles with a 10,000 pound bombload, a cruising speed of more than 450 knots, a combat altitude greater that 35,000 feet and a takeoff distance of less than 6000 feet.
In the meantime, the Boeing Airplane Company, now freed up by the cancellation of the XB-55 project, began to study the possibility of a high-performance medium bomber. Performance objectives included a combat radius of 3000 miles at an altitude of 50,000 feet. The aircraft would be capable of a supersonic speed of Mach 1.3 within 200 miles of the target. After looking at several different configurations, the Air Force selected the Boeing Model 484-405B as having the highest potential. The 484-405B was a fairly conventional design, with a low aspect ratio, high-mounted wing with a sweep of 47 degrees. A bomb bay similar in size to that of the B-47 would be provided. Gross weight was 200,000 pounds. The aircraft was to be powered by four Pratt & Whitney J57-P-5 afterburning turbojet engines. The engines were to be mounted side-by-side, two in the inboard section of each wing. Because the wing had to be thin in order to make it possible to achieve supersonic performance, all of the fuel had to be housed entirely within the fuselage. The fuselage housed a pressurized cabin for a crew of three. A remotely-controlled tail turret was to be fitted.
The Convair Aircraft Corporation also submitted a design to meet the requirement. In January of 1950, Convair, as part of its work on GEBO II, began to explore a parasite concept. They proposed a fairly small delta-winged aircraft which would be carried part-way to its target underneath a B-36. The aircraft was to carry a two-man crew and would have four turbojet engines. Very early on, however, the parasite concept was abandoned, to be replaced by an aircraft with conventional landing gear.
By the end of 1950, the Bombardment Branch of the Air Materiel Command's Aircraft and Guided Missiles Section began to prepare a detailed military specification for both the Boeing and Convair proposals. Based on the AMC proposal, which was in turn based on input from both the Boeing and Convair design studies, requests were made for funds for beginning projects. The supersonic bomber was now officially a part of the Air Force's future plans.
On January 26, 1951, following the completion of the detailed study, Convair proposed that it develop a long range supersonic reconnaissance bomber. The project was given the number MX-1626 by the AMC under contract AF33(038)-21250. In February, the competing Boeing project was given a development contract by the AMC under the designation MX-1712 and contract AF33(038)-21388. Boeing's contract called for Phase I development of two bomber/reconnaissance aircraft through wind tunnel testing, engineering, and mock-up. Initial flight dates for both designs were tentatively set for late 1954.
On February 1, 1952, the USAF issued General Operational Requirement SAB-51, where SAB stood for Supersonic Aircraft Bomber. It called for a multi-mission strategic reconnaissance bomber capable of carrying 10,000 pounds of bombs. It had to be capable of operating in all weather conditions, and had to be able to achieve a combat radius of 5000 miles with a single outbound inflight refuelling. It had to be capable of supersonic performance at altitudes of 50,000 feet or more and had to be able to achieve a high subsonic performance at lower altitudes. It was considered important that the aircraft be fairly small, since this would reduce the radar reflectivity and make the aircraft harder to detect. The Air Force wanted production to begin within five years.
On February 26, 1952, the SAB-51 GOR was revised in a document which came to be known as Directive Number 34. It was conceded that it was unrealistic to expect the rapid development of a high-altitude, long-range supersonic bomber that could also be suitable for low-altitude high-speed missions. Consequently, the low-altitude performance requirement was dropped. Following discussions with the Air Council and representatives of the ARDC, SAC, the Rand Corporation, and the Scientific Advisory Board, the Air Force endorsed this recommendation, and the revised SAB became formalized on September 1, 1952 as SAB-52-1. However, the Air Force still wanted the aircraft by 1957.
At the end of February 1952, General J. W. Sessums, ARDC Deputy for Development recommended that it would be better to forego the traditional industry-wide competition that would ordinarily be held for the supersonic bomber project. Time and money would be saved if contractors could be selected on the basis of the proposals already submitted. Although the AMC felt that the Boeing and Convair proposals offered the best hope for a supersonic bomber, the AMC had requested informal proposals from other manufacturers, including Douglas, Lockheed, Martin, and North American. However, only two of the last four companies actually submitted proposals, and these were not very interesting. Shortly thereafter, the Wright Air Development Center endorsed this strategy and called for a competition between Boeing and Convair, the only two companies to have submitted proposals that were of any significant interest.
The Air Force was now committed to the advanced bomber project, and placed heavy emphasis on the MX-1626 and MX-1712 programs. It requested that two parallel Phase 1 projects be initiated, thus engaging Boeing and Convair in an official competition. It was anticipated that contracts would be issued to both competetors in the fall of 1952 for detailed designs and mockups, followed by the selection of a winning design in February or March of 1953. The emphasis would continue to be on minimum size and maximum altitude and speed performance.
The financing of the Phase I development of two parallel projects was extremely difficult to support, especially during a period of financial austerity. The Boeing MX-1712 program had benefited somewhat from the XB-55 cancellation, which freed up some Boeing developmental funding for the new project, but Convair's MX-1626 was experiencing a severe funding problem. In late February, the MX-1626 program was almost cancelled due to the lack of funds, and the project remained in some danger until May 15, when enough additional funds were obtained to keep the project going.
Directive 34 had also dictated that the project use the weapons system concept, in which the equipment, weapons, electronics, and components of the aircraft would be developed as an integrated whole to ensure that each component would be compatible with the others. By mid-1952, both Boeing and Convair had made considerable progress in bringing their projects into compliance with the weapons system philosophy. In the process of making their designs conform with the requirements of Directive 34, Convair's MX-1626 was now known as MX-1964 and Boeing's MX-1712 was now called MX-1965. The USAF designations B-58 and B-59 were tentatively assigned to the two competing projects, even though no production orders were yet forthcoming.
In the summer of 1952, the Wright Air Development Center concluded that a less costly alternative would be to select just one of the two competitors even before the design and mockup stage was reached. The small bomber concept was endorsed by the Air Force Council and by General Hoyt S. Vandenberg, who was Chief of Staff of the Air Force. However, General Curtis LeMay, head of the Strategic Air Command, generally favored the development of larger bombers with longer ranges. SAC felt that high performance alone would not necessarily assure mission success, and that the small supersonic bomber's lack of range would prevent it from operating without midair refueling from most forward bases. Despite SAC's objections, the Wright Air Development Center recommended that the Boeing/Convair competition be stopped. Even though the Air Force thought that Convair's estimates of the MX-1964's supersonic drag and gross weight were overly optimistic, the Air Force felt that the Convair design was superior to the Boeing proposal. It was concluded that the Boeing design would offer insufficient supersonic capabilities, and on November 18, 1952, General Vandenberg formally announced that Convair was the winner of the contest. All work on the competing B-59 project was stopped.