The 1st Air Fleet (第一航空艦隊, Daiichi Kōkū Kantai), also known as the Kidō Butai ("Mobile Force"), was a combined carrier battle group comprising most of the aircraft carriers and carrier air groups of the Imperial Japanese Navy (IJN) during the first eight months of the Pacific War.

At the time of its attack on Pearl Harbor, in December 1941, the 1st Air Fleet was the world's largest fleet of aircraft carriers.

In its second generation, 1st Air Fleet was a land-based fleet of "kichi kōkūtai" (base air unit(s)).

In 1912, the British Royal Navy had established its own flying branch, the Royal Naval Air Service (RNAS). The IJN was modeled on the Royal Navy and the IJN Admiralty sought establishment of their own Naval Air Service. The IJN had also observed technical developments in other countries and saw military potential of the airplane. In 1913, the IJN seaplane carrier Wakamiya was converted into a seaplane tender and aircraft were purchased. The 1st and 2nd Air Fleet were to be the primary attack force of the IJNAS.

The Japanese carriers' experiences off China had helped further develop the IJN's carrier doctrine. One lesson learned in China was the importance of force concentration and mass in projecting naval air power ashore. Therefore, in April 1941 the IJN formed the 1st Air Fleet to combine all of its fleet carriers under a single command. The IJN centered its doctrine on air strikes that combined the air groups within carrier divisions, rather than each individual carrier. When more than one carrier division was operating together, the divisions' air groups were combined with each other. This doctrine of combined, massed, carrier air attack groups was the most advanced of its kind of all the world's navies. The IJN, however, remained concerned that concentrating all of its carriers together would render them vulnerable to being wiped out all at once by a massive enemy air or surface strike. Thus, the IJN developed a compromise solution in which the fleet carriers would operate closely together within their carrier divisions but the divisions themselves would operate in loose rectangular formations, with approximately 7,000 metres (7,700 yd) separating the carriers from each other.

Although the concentration of so many fleet carriers into a single unit was a new and revolutionary offensive strategic concept, the First Air Fleet suffered from several defensive deficiencies which gave it, in Mark Peattie's words, a "'glass jaw': it could throw a punch but couldn't take one." Japanese carrier anti-aircraft guns and associated fire control systems had several design and configuration deficiencies which limited their effectiveness. The IJN's fleet combat air patrol (CAP) consisted of too few fighter aircraft and was hampered by an inadequate early warning system, including a lack of radar. Poor radio communications with the fighter aircraft inhibited effective command and control of the CAP. The carriers' escorting warships were deployed as visual scouts in a ring at long range, not as close anti-aircraft escorts, as they lacked training, doctrine, and sufficient anti-aircraft guns. These deficiencies would eventually doom Kaga and other First Air Fleet carriers.

The First Air Fleet (Dai-ichi Kōkū Kantai) was a major component of the Combined Fleet (Rengō Kantai). When created on 10 April 1941, it had three kōkū sentai (air flotillas; in the case of aircraft carriers, carrier divisions): On that date, First Kōkū Sentai consisted of Akagi and Kaga and their aircraft units. Later that spring, a number of destroyers were added. On 10 April 1941, Second Kōkū Sentai comprised Sōryū, Hiryū and the 23rd Kuchikutai (Destroyer Unit). Fourth Kōkū Sentai consisted solely of light carrier Ryūjō and her aircraft unit, until two destroyers were added in August. (At its inception, First Air Fleet did not include Third Kōkū Sentai and it did not include it on 7 December 1941. Third Kōkū Sentai (3rd Carrier Division, see table below) was attached to First Fleet, as distinct from First Air Fleet. On 1 April 1942, Third Kōkū Sentai was disbanded.Imperial Flattops) See the table titled "Transition", below.

When formed on 10 April 1941, First Air Fleet was a naval battlegroup with the single most powerful concentration of carrier-based aircraft in the world at the time. Military historian Gordon Prange called it "a revolutionary and potentially formidable instrument of sea power."