In the years before they flew their 37-hour mission to strike Iran’s nuclear site at Fordo, the U.S. Air Force pilots spent at least 24 hours straight in a B-2 bomber flight simulator that is a replica of their cockpit.

In the days or weeks leading up to the mission, they most likely ran simulated runs on a target made to look like the heavily fortified site buried deep in a mountain.

Almost everything about the mission, flown from Whiteman Air Force Base in Missouri, would feel the same with just a couple of big differences, said retired Lt. Gen. Steven L. Basham, who flew the plane in training and combat missions for nine years.

In the real mission, flown in the early hours of Sunday morning in Iran, the pilots would “feel the clunk” of their weapons bay doors opening, briefly changing the shape of the stealth plane and potentially exposing it to enemy radar.

The B-2s that attacked Fordo were each carrying two Massive Ordnance Penetrator bombs designed to disable the deeply buried target. When the two-person crews released their payload, weighing a total of 60,000 pounds, their B-2 most likely surged briefly upward, General Basham said.

For the pilots, it was almost certainly a new feeling.

Other bombers in the American arsenal, such as the B-1 and B-52, played big roles in the wars in Iraq and Afghanistan, dropping huge numbers of bombs in support of ground troops. But the B-2 — the most expensive plane in history, at $2.2 billion a copy — played a much more specialized role.