The XW-35 was designed from the outset as a thermonuclear warhead for the first generation of ICBMs. Development was driven by the development of the Atlas missile

Atlas (missile)

The SM-65 Atlas was the first intercontinental ballistic missile developed and deployed by the United States. It was built for the U.S. Air Force by Convair Division of General Dynamics at the Kearny Mesa assembly plant north of San Diego, California...

, and when the accuracy of the Atlas was shown to be inferior to predictions the XW-35 design had to be altered to give a higher yield

Nuclear weapon yield

The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...

. By March 1958 the development of the XW-35-X1 was lagging. The XW-35-X1 was probably the device tested in the Koa shot of the Operation Hardtack I

Operation Hardtack I

Operation Hardtack I was a series of 35 nuclear tests conducted by the United States in 1958 in the Pacific Ocean.Operation Newsreel was a series of three high-altitude nuclear tests conducted as part of Hardtack I. The individual tests in the series were Orange‎, Teak‎ and Yucca‎.-Test Blasts:-...

 series (May 1958). Predicted yield was 1.75 megatons; the actual yield was 1.37 megatons with the shortfall due to poor burning of the secondary. By this time the Air Force had designated the XW-49, a simple modification of the TX-28 already successfully tested during the Redwing series. By August 1958 the first dedicated missile warhead design, the W-35 had been cancelled in favor of the W-49.