Well, the US military had NO brief for a dedicated attack helo before Vietnam - they hadn't even armed transport helos yet.
It was as a result of the first use of helos in the MAC-V days (1961-62) that field-modified UH-1s (HU-1s at the time) became the first armed support helos in the US military.
Bell had been exploring the concept since the late 1950s, paying attention to the French having armed helos during the Algerian war (eerily similar to the later Vietnam war), and when reports came back of the field-armed UH-1s, Bell developed a mockup of a dedicated gunship helo (D-225) that was very similar to the eventual AH-1. Bell showed this to the Army in June 1962, convincing the Army to fund a development prototype in Dec 1962. When this (the Sioux Scout) flew in July 1963, the army then launched the AAFSS competition that produced the AH-56.
Even with this, Bell had to use its own funds to create the first AH-1 demonstrator (Model 209), starting in January 1965 and culminating on 7 September 1965 with its first flight in front of 20 surprised US Army officials (like the rest of the Army they were completely unaware of what Bell was doing). The Army then sent a Request For Proposal to 5 companies (including Bell) for a dedicated attack helo - with its head start, Bell won in April 1966, and the first AH-1Gs were delivered in June 1967, arriving in Vietnam on 30 August 1967.
Without combat experience in Vietnam, how long would it take the US Army to even formulate the need for a dedicated armed helo... or would they, like the British (armed Lynx), Germans (Bo-105) and French (Alouette II/III & Gazelle), simply arm Hueys with guns, rockets, and anti-tank missiles throughout the 1960s?
Would there be a different "low-intensity conflict" to show the need for a dedicated design? Or would the AAFSS competition simply never occur?
