A heavy bomber is a bomber aircraft of the largest size, and typically longest ranges. The term was used primarily prior to and during World War II, when engine power was so scarce that designs had to be carefully tailored to their missions. The heavy bomber was generally considered to be any design that delivered 8,000 lb (4 t) of bombs or more on distant targets, with medium bombers having loads of 4,000 to 8,000 lb (2 to 4 t), and light bombers 2,000 to 4,000 lb (1 to 2 t). These distinctions were already disappearing by the middle of WWII, when the average fighter aircraft could now carry a 2,000 lb (1 t) load and the "light" designs had now largely taken over the missions formerly filled by "mediums".
Heavy bombers furthermore usually had a very heavy defensive armament. British designs commonly had three gun turrets with a total of 8 guns. US designs, built around formation flying, had upwards of 10 machine guns and / or cannons in both turrets and flexible mounts of lesser arcs to deliver the best curtain of fire. Positions for these guns included tail turrets, side gun ports (often a simple window with a .50 caliber machine gun sticking out of it), dorsal (top-of-aircraft) turrets, and ventral (under-aircraft) gun positions which on American bombers were often ball turrets. All of these machine guns enabled heavy bombers to defend themselves reasonably well after they had passed the maximum range of their fighter escorts. British heavy bombers flew more independently by night and evasive maneuvring (the corkscrew) played as much a part in defence. The manned lower-ball gun turret on the B-17 and B-24 bombers were a marked improvement over the previous remote belly turret; the ball gun turret rotated a full 360 degrees with an effective 90-degree elevation and a range of one thousand yards.
After World War II the term saw some limited use to describe bombers dedicated to the strategic role, but soon these were being referred to as strategic bombers, while every other design became a tactical bomber. The general utility of a manned heavy bomber has been greatly degraded with the introduction of more accurate precision-guided munitions (or "smart bombs"), but the B-52 continues to fill a role when the target requires a massive number of bombs to be dropped. Current in service heavy bombers such as the B-2A "Spirit" also participate in launching cruise missiles, such as the ALCM/CALCM.