The night Truman was sworn in as Roosevelt’s successor he noted in his diary that Stimson told him the U.S. was “perfecting an explosive great enough to destroy the whole world.”

And if you wanted to know what Truman thought of not being told as vice president. (from the same link)

On June 17, Truman received a phone call from Stimson, who told him that the Pasco plant was “part of a very important secret development.” Fortunately, Stimson did not need to explain further: Truman, a veteran and a patriot, understood immediately that he was treading on dangerous ground. Before Stimson could continue, Truman assured the secretary “you won t have to say another word to me. Whenever you say that [something is highly secret] to me that’s all I want to hear. If [the plant] is for a specific purpose and you think it’s all right, that’s all I need to know.” Stimson replied that the purpose was not only secret, but “unique.”