In the mid-1930s, Northrop began work on the BT-1, a carrier-based dive bomber. By 1937, Douglas Aircraft had taken over Northrop and carried the project forward. The BT-2, a modified version of the BT-1, became the foundation for the Douglas SBD Dauntless. Ed Heinemann and his design team developed the new plane with a more powerful Wright Cyclone engine, improved handling, and unique perforated dive brakes to keep the aircraft stable in steep dives. Built at Douglas plants in El Segundo and Oklahoma City, the SBD entered service in 1939. The Marine Corps received the first SBD-1s in 1940, while the Navy began using the SBD-2 in early 1941 to replace older carrier aircraft. As war approached, the SBD continued to evolve. The SBD-3, introduced in 1941, carried more armor, self-sealing fuel tanks, and four machine guns. The SBD-4 brought an upgraded electrical system, and the widely produced SBD-5 came with a stronger 1,200 hp engine and greater ammunition capacity. More than 2,400 of this version were built, many at the Douglas Tulsa plant. The final model, the SBD-6, appeared in 1944 with further refinements, though production ended that same summer as newer designs entered service. During the Pacific War, the Dauntless quickly earned its reputation. Its combination of range, maneuverability, and precise dive-bombing ability made it the Navy's main carrier-based bomber from 1940 to 1944. At the Battle of Midway in June 1942, SBDs dealt decisive blows to four Japanese carriers, shifting the balance of the war. Despite being slower than newer planes, its reliability and effectiveness earned it the nickname 'Slow But Deadly.' Even when officially replaced by the Curtiss SB2C Helldiver, many squadrons kept the SBD in service, and both types flew side by side until the war's end. The Dauntless also saw service beyond the Navy. The Marine Corps flew it from land bases and carriers, and the U.S. Army Air Forces adopted a modified version called the A-24 Banshee. Without carrier gear, the A-24 struggled in combat due to limited speed and weak defenses, leading to its reassignment to non-combat duties by 1943. Other operators included New Zealand, Free France, and Mexico, though the aircraft was soon overshadowed by faster fighters like the Corsair. In total, 5,937 SBDs were built, with nearly 1,000 used by the Army. Despite its eventual obsolescence, the SBD Dauntless remained one of the most important dive bombers of the Second World War.