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In July 1921, the Army Air Service and the US Navy conducted a series of bombing tests off Cape Henry, led by General Billy Mitchell. The targets included demobilized American and former German warships, including the old battleship Iowa, the cruiser Frankfurt, and finally Ostfriesland on 20 July. At 13:30 ET, the first attack wave, armed with 230 lb (100 kg) bombs, struck the stationary ship. Eight of thirty-three bombs found their mark, after which the ship was inspected. The second wave was also armed with 230 lb bombs, and the third and fourth carried 600 lb (270 kg) bombs. Five 600 lb bombs found their mark, but little damage was done to the ship's topside. The bombs that nearly missed the ship, however, had done significant underwater damage to the hull, which allowed some flooding and created a list of five degrees to port and three additional feet of draft at the stern. The bombing schedule was interrupted by a storm in the late afternoon.
Early on the morning of 21 July, the fifth wave of bombers began their attack. At 08:52, the first Army bomber dropped a 1,000 lb (450 kg) bomb that hit the ship; four more bombers followed and scored two further hits. Inspectors again went aboard Ostfriesland following the fifth attack and noted that the hits had not seriously damaged the ship, though one had created a large hole on her starboard side that allowed further flooding. By noon, she was down five feet at the stern and one foot at the bow. At 12:19, the next attack wave, equipped with 2,000 lb (910 kg) bombs, struck. Six bombs were dropped, none of which hit, though three detonated very close to the hull. At 12:30, Ostfriesland began to sink rapidly by the stern and the list to port increased dramatically. At 12:40, the ship rolled over and sank. The results of the tests were widely publicized and Mitchell became both a national hero and the "infallible prophet of aviation".
The Navy had allocated another vessel, the pre-Dreadnought battleship Alabama, to the Army for bombing tests. Mitchell’s bombers sank it in the Chesapeake Bay Sept. 27, 1921. After that demonstration, the First Provisional Air Brigade was disbanded.
In another series of tests in 1923, Mitchell and the Air Service sank the surplus battleship "Virginia" and severely damaged the battleship "New Jersey" in operations off Cape Hatteras.