The Operation Sandstone nuclear tests in April and May 1948 had demonstrated improved designs, with the X-Ray and Yoke tests having yields of and respectively—nearly twice that of the older Mark 3 Fat Man devices in the inventory. The new Mark 4 nuclear bomb, which entered service in March 1949, was a more practical piece of ordnance than its predecessor, and its composite uranium-plutonium core made more economical use of the available fissile material. In May 1948, the Los Alamos Scientific Laboratory commenced work on the design of the Mark 5 nuclear bomb, a smaller and lighter weapon. The development of Soviet atomic bombs provided the impetus for the development of even more destructive thermonuclear weapons. On 31 December 1949, the Strategic Air Command had 521 B-29s, B-36s and B-50s capable of delivering atomic bombs. It was estimated that SAC bombers would suffer 35 percent casualties at night, and fifty percent if missions had to be conducted in daylight. The delivery of the 292 atomic bombs called for by the Offtackle plan was regarded as practical, but there would be no ability to launch follow-up raids. A jet bomber, the Boeing B-47 Stratojet was under development, but would not become operational until 1953. The concept of nuclear deterrence did not figure in the war plans; nuclear weapons were seen purely as weapons of war.
