From what little I know about the planning of D-Day they did not expect it to succeed. If this is true, then why did they proceed? Was it the least of bad options?
I don't think that there were all that many who expected failure. Are there particular facts or statements that led to your conclusions?
(There were many who recognized that it entailed significant risks, and it always makes sense to plan for every contingency. Even Eisenhower had his concession speech pre-written.)