The idea that led to 9/11 as narrated by al-Qaida: The original idea came from an Egyptian pilot who migrated to Peshawar and stayed at one of the Jihadi guest houses. In an informal sitting, he would first air his idea to hijack airplanes to strike iconic building in the US

This idea was shared with AQ's military leader at the time Abu Ubaydah al-Banshiri (also Egyptian) and later recounted by senior AQ figures Abu Hafs al-Masri and Abu al-Khayr. When in Sudan, UBL arranged for two AQ members to attend flight training.

However, plans speeded up after Khalid Sheikh Muhammed approached AQ to air his own idea of hijacking airplanes. AQ was located in Sudan at the time and the country was not considered an appropriated launching pad for such a major attack.

It was thus when AQ had relocated back to Afghanistan in 1996 that the plans started to take form with KSM as the main entrepreneur. The rest is history.

As said, this is AQ's own version. What they forgot to mention is that the majority of the AQ leadership initially actually opposed the attack.