What was the difference between the 8th and 9th Air Force?

The US 8th Air Force was established in Britain in 1942.  As its strength was growing a large part was diverted to form the 12th Air Force in North Africa.  From spring 1943, development of the 8th resumed, as a strategic bomber force.

From October 1943 the HQ of the 9th Air Force, previously base in Egypt, was transferred to the UK to form a tactical air force.  The 9th provided support for ground forces in North West Europe.  Some of its units came form the 8th, others direct from the US.

Also in 1943, other units were diverted from the 8th to create the 15th Air Force.  This was a strategic force formed in Italy to complement attacks on Germany by the 8th.

Nevertheless by 6 June 1944, the day of the Allied invasion of Europe, the 8th had 3,000 heavy bombers, 1,600 fighters and 175 reconnaissance aircraft: the largest air striking force ever committed to battle.  

Alongside it on that day and soon to move to the Continent was the 9th with a further 1,050 medium bombers, 1,900 fighters 1,400 troop carrier aircraft and 175 reconnaissance aircraft.