bombing, an anti-shipping technique used by the British and Germans. Flying only a few dozen feet above the sea toward their targets, bombers would release their bombs which would then, ideally, ricochet across the surface of the water and explode at the side of the target ship, under it, or just over it. A similar technique was mast-height bombing, in which bombers would approach the target at low altitude, , at about , and then drop down to mast height, at about from the target. They would release their bombs at around , aiming directly at the side of the ship. The Battle of the Bismarck Sea would demonstrate that this was the more successful of the two tactics. The two techniques were not mutually exclusive: a bomber could drop two bombs, skipping the first and launching the second at mast height. In addition, as regular bomb fuses were designed to detonate immediately on impact, which would catch the attacking aircraft in its own bomb blast at low altitude attacks, crews developed a delayed-action fuse. Practice missions were carried out against the wreck of the , a liner that had run aground in 1923. In order for bombers to conduct skip or mast-height bombing, the target ship's antiaircraft artillery would first have to be neutralized by strafing runs. For the latter task, Major Paul I. "Pappy" Gunn and his men at the 81st Depot Repair Squadron in Townsville, Queensland, modified some USAAF Douglas A-20 Havoc light bombers by installing four machine guns in their noses in September 1942. Two fuel tanks were added, giving the aircraft more range. An attempt was then made in December 1942 to create a longer range attack aircraft by doing the same thing to a B-25 medium bomber to convert it to a "commerce destroyer", but this proved to be somewhat more difficult. The resulting aircraft was nose-heavy despite added lead ballast in the tail, and the vibrations caused by firing the machine guns were enough to make rivets pop out of the skin of the aircraft. The tail guns and belly turrets were removed, the latter being of little use if the aircraft was flying low. The new tactic of having the B-25 strafe ships would be tried in this battle. The Fifth Air Force had two heavy bomber groups. The 43rd Bombardment Group was equipped with about 55 Boeing B-17 Flying Fortresses. Most