The hand grenade had origins in ancient history when it was learned that flammable contents could be contained in a stone or ceramic case and hurled at the enemy for tremendous area damage while also doubling as a psychological factor in combat. World War 1 (1914-1918) excelled its use in infantry warfare and, by the time of World War 2 (1939-1945), the hand grenade was a standard infantry-level offensive-minded weapon adopted by many of the leading nations of the conflict.
The Empire of Japan employed several different kinds of hand grenades during the period, some like the Type 10 emerging from post-World War 1 development while the Type 91 and Type 97 joined during the interwar years. The Type 99 then followed in 1939 to help shore up the issues that crept up out of the operational use of the Type 97. The Type 97 had origins in the earlier Type 91 which held a rather lengthy delay before detonating. The Type 97 was only a slightly improved form in that it featured a shortened fuse timer but lacked the destructive capabilities of hand grenades employed by the Allies during the same time. It was also incapable of being fired from standard issue "grenade dischargers" like the Type 100 series - these weapons equivalent to personal mortar systems though with far less lethality.
With this origination, the Type 99 appeared itself as an improved form of the Type 97 and was issued to elements of both the Imperial Japanese Army (IJA) and Imperial Japanese Navy (IJN). Design was headed by the Army Technical Bureau and featured a few changes over that of the earlier Type 97. Its body no longer carried the segmentation popular with grenades of the war - instead the case was smooth. A similar cylindrical shape was used and filling was 58 grams of cast piric acid. A pyrotechnic delay of 4-5 seconds made up the detonation mechanism. Beyond the traditional hand-thrown action, the Type 99 held an inherent capability to be fired from the Type 100 discharger series or from the muzzle of a standard Japanese service rifle by way of an adapter. The activation sequence was also streamlined in the Type 99 - the operator pulling the safety pin and then striking the fuse on a hard surface. The grenade was then thrown at the enemy in the usual way.
Service of Type 99s spanned from 1939 to 1945. They were first encountered by American troops during the Battle of Kiska as part of the Aleutian Islands Campaign of 1942-1943. For this reason, the grenade came to be known in American lingo as the "Kiska Grenade". Its operational use more or less ended immediately with the Japanese surrender of August 1945.