An important feature of the Lancaster was its unobstructed 33 ft. long bomb bay. At first, the heaviest bomb carried was the 4,000 lb. high capacity HC "Cookie". Later, bulged doors allowed the aircraft to carry 8,000 lb. and 12,000 lb. "Cookies". Lancasters also carried a variety of smaller weapons, including the Small Bomb Container that held incendiary and explosive incendiary bomblets; 500 lb and 1,000 lb General Purpose High Explosive bombs; 1,850 lb parachute deployed magnetic or acoustic mines, or 2,000 lb armour-piercing bombs; 250 lb Semi-Armour-Piercing bombs; and 250 lb or 500 lb anti-submarine depth charges.
MK.IV 4000 lb High Capacity Bomb
The 4,000 pound HC bombs were commonly referred to by the RAF as "Cookies". They were often used to open up (blow roofs off and windows in) buildings in the target area. This allowed for incendiary bombs dropped in the following attack waves to have ample fuel in which to ignite and burn.