Boeing XB-55 (company designation Model 474) was a proposed Boeing aircraft designed to be a strategic bomber. The XB-55 was intended to be a replacement for the Boeing B-47 Stratojet in United States Air Force (USAF) service. Design and development The XB-55 concept was contained in a Request for Proposal (RFP) issued by the United States Air Force in October 1947, two months before the first flight of the XB-47 prototype. Several United States manufacturers responded to the RFP. Boeing was selected from among this group and given a contract on 1 July 1948 to conduct further engineering studies. Boeing's initial approach was to mount four turboprop engines on an airframe similar to its B-47: the wing would have less sweepback; the Allison T40-A-2 engines would drive three-blade contra-rotating propellers, i.e., six blades per engine; the engines were to be mounted in nacelles hung from the wings, two per side; the landing gear was to be similar to the B-47's tandem gear with outriggers retracting into the outboard engine nacelles. The XB-55 had a projected top speed of and a cruising speed of , with a maximum weight of , a wingspan of , and length of . There was a major disagreement between the engine manufacturer and the propeller manufacturer over whether the Allison T40-A-2 driveshaft was strong enough to take the forces caused at high revolutions per minute of the propellers. Allison was predicting that it would be at least four years before a successful powerplant would be delivered. In October 1948, a conference in Dayton, Ohio was addressing the problems of the XB-55 when it was proposed over lunch that the XB-52 (Boeing Model 464), which until that point had been planned with turboprop engines, could be equipped with the forthcoming Pratt & Whitney J57 turbojet engines. Within a week, it was clear that not only would the XB-52 outperform the XB-55, it could be flying at least a year before the XB-55 could be expected to have reliable engines. Also bearing on the decision to abandon the XB-55 program were government funding constraints and the growing realization that the B-47 was becoming more successful than first projected. On 29 January 1949, the Air Materiel Command was directed to cancel the Boeing XB-55 contract. Under a revised contract, the Boeing Project 474 was converted into the Boeing Project 479, which included a study of using six J40