According to the New York Times on Monday, federal officials and private experts said that the re-design marked a "fundamental shift" in philosophy.
The newspaper said bomb makers in the past had sought to use the latest technologies and innovative methods to make warheads that were lightweight, powerful and sometimes so small that a dozen could fit atop a missile.
Now, the newspaper said, American designers are looking at how to make arms that are more robust, in part to avoid the uncertainties and deteriorations of nuclear old age.
The article added that so far the effort has required only $9 million for warhead designers at the nation's three nuclear weapon laboratories.
Bomb experts are looking at secret arms data for clues about how to achieve the new reliability goals.
The initial programme, which involves fewer than 100 people, is expected to grow.
The programme could produce finished designs in the next five to 10 years and possibly culminate in prototype warheads.