The Douglas A-26 Invader saw most combat during the Second World War against the Germans, serving in significant numbers with the Ninth Air Force. The aircraft's combat debut in the Pacific had been unimpressive – four early B-26s with the original flush canopy and their under-wing gun pods removed to improve their performance had been tested by the 13th Bombardment Squadron of the 3rd Bombardment Group on New Guinea, and had made a very bad impression, mostly because of the poor visibility from the cockpit.
Douglas needed better results from the Invader's second combat test, which was to be carried out by the 553rd Bombardment Squadron of the 386th Bombardment Group, part of the Ninth Air Force, which by the late summer of 1944 was heavily involved in the campaign in north-western Europe. Eighteen A-26s were sent to Britain, and between 6 and 19 September took part in eight medium level bombing missions, starting with an attack on Brest.
The Ninth Air Force report was overwhelmingly positive. The A-26 could carry more bombs than the A-20, and had a larger combat radius, better single engine performance, flying characteristics and manoeuvrability than either the A-20 or the B-26. No aircraft were lost on the eight test missions, and the Ninth Air Force announced that it was happy to replace all of it’s A-20s and B-26s with the A-26 Invader.
The Ninth Air Force became the main wartime user of the A-26, and eventually five bombardment groups saw some action against the Germans. The first to enter combat were the 409th and 416th Bombardment Groups, which received their A-26s in time to use them against German communications during the battle of the Bulge. The 386th Bombardment Group entered combat soon after the end of the battle, and in April the 391st became the last group to convert entirely to the A-26, using against them against the German transport network. The 410th Bombardment Group was in the process of converting to the A-26 when the war ended, although it did receive a small number in January which it used as pathfinders to guide the group's A-20s and B-26 Marauders to their targets.
In Italy the Twelfth Air Force's 47th Bomb Group also received the A-26, starting in January 1945. Once again they were used against German transport links, but also for direct support and interdiction against tanks and troop concentrations in the Po valley in the final campaigns in Italy.
Finally the 492nd Bomb Group received a small number of A-26s, which it used on carpetbagger missions behind German lines, dropping agents and supplies to support clandestine operations and resistance groups,