I have it on good authority that NO 4 engined bombers ever did a mission under 10k.

B-17's WERE used in the pacific for skip bombing attacks on the IJN fleet. There are even books written on it by people that were there. They would approach at about 200 feet (at night or in bad weather) and skip bomb modified 1000 pounders into the ship. Here is a publishers note from the book "Skip Bombing" by James T. Murphy:
"Murphy was one of a very small number of volunteer pilots who, with their flight crews, started bombing at low altitudes in B-17 flying fortresses in the Southwest Pacific. The aircraft were flown at a 200-foot altitude and at 250 miles per hour at night. One-thousand pound bombs, equipped with four-to-five second fuses, were dropped from the B-17s. On March 3, 1943, the Japanese made a desperate move to re-supply their forces on New Guinea. Twenty-two cargo, transport, and war ships proceeded toward New Guinea using bad weather for cover. They were found in the Bismarck Sea. The Allied Air Forces--using skip bombing--sank all twenty-two Japanese ships. Murphy was credited with sinking nine Japanese ships during his year of combat, including one in the Bismarck Sea battle. Skip bombing became a tactic that helped the U.S. win the war in the South Pacific."
It wasnt done in high numbers, but it was done.
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Edgar