K-Rations: "Food for Fighters" circa 1943 US Office of War Information World War II

The lightweight, easy-to-carry K Ration was designed for the assault phase of combat operations, first put to use by Army paratroopers. To the amazement of its developers, it became an overnight success and was soon adopted for all-service use.

The Quartermaster Corps purchased its first million K-Rations in May 1942. In the peak year, 1944, they procured more than 105 million. Contrary to several long-standing myths, there is no special significance attached to the letter "K." It was simply picked to make it phonetically distinguishable from C- and D-Rations.

Although other related items influenced its development, the actual prototype of the K Ration was a pocket ration for paratroopers developed by the SR&DL at the request of the Air Corps early in the war. Two original samples (one with pemmican biscuits, a peanut bar, raisins, and bouillon paste; the other with pemmican biscuits, a small D bar, a meat preparation, and beverage (powder) evolved into the one-package breakfast-dinner-supper combination used first by paratroopers.

The three-meal combination contained such common units as pemmican biscuits and gum. In addition, the breakfast unit furnished malted milk tablets, canned veal loaf, soluble coffee, and sugar; the dinner package had dextrose tablets, canned ham spread, and bouillon cubes; and for the supper unit there were the D bar chocolate, sausage, lemon powder, ant sugar. The Army quickly noted the success of the new ration with the paratroops and in 1942 the item was adopted for all-service use as Field Ration, Type K. The instantaneous success of the ration with attendant popular publicity, was a source of amazement to the developers.
http://olive-drab.com/od_rations_k.php
https://en.wikipedia.org/wiki/K-ration