Thank you for the historical insight. I probably would have to study years to gain such knowledge from books. I have an idea that I'd like to share...

The big aircraft that were used in the attacks allowed the delivery of explosive energy on the order of multi-kilotons of TNT. This energy breaks down as follows:

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Fuel content of planes: 1,500,000,000,000 calories
Kinetic Energy of impacts: 710,000,000 calories
Potential energy of buildings: 400,000,000,000 calories
Total of above: 1,900,710,000,000 calories
Energy released in WTC event : 1.9 kilotons
Energy released by building collapse, on Richter scale: 3.5
Hiroshima bomb: 13 kilotons
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Large aircraft today are essentially mini-nuclear bombs with wings, able to be hijacked by suicide bombers and aimed at any military or civilian target. But what if the aviation fuel hadn't burned? The kinetic energy of each aircraft impact would have been far smaller, equivalent to just 187 kilograms (411 lbs) of TNT. It is possible that the buildings would not have collapsed, and the death toll would have been limited to the number of passengers on the plane and those in the section of the building hit. But how to keep the fuel from burning?

For years DaimlerChrysler Aerospace Airbus corporation has been funding a R&D project called Cryoplane. This aircraft uses liquid hydrogen to power twin turbofan engines. So far it has been successful in flight tests, with modifications to flight plans, although it is not yet cost-competitive with current Airbus vehicles that burn fossil fuels. The main drawback is the need for cryogenic tanks to store the liquid/slush hydrogen, whose shapes are restricted to spherical, or short cylindrical, tanks to prevent excessive heating and boil-off of the hydrogen. This means that the fuel cannot be stored in the wings. The bulky hydrogen tanks must be located either inside the fuselage or hang underneith the wings. Either case subtracts cargo capacity or aerodynamic efficiency which directly affect the performance and cost-competitiveness of the aircraft. Liquid hydrogen is also flammable, just like fossil fuel.

Reference:
Flug Revue

The key to overcoming this obstacle is a new storage system for the hydrogen. The ideal would negate the need for cryogenic tanks and allow fuel to be stored in the wings. I now introduce liquid sodium borohydride. This awesome storage medium for hydrogen has been demonstrated by Millennium Cell. It is a liquid at room temperature and pressure, stores 66 percent as much hydrogen per litre as liquid hydrogen, has the same mass-density as water, can be pumped into and out of a tank, and can be re-charged with hydrogen over and over again.

Reference:
Millennium Cell

Now here is the interesting part. The fuel is not flammable, except in the presence of a rare-earth catalyst. If large aircraft were to use this fuel instead of kerosene they would no longer be effective weapons against us. They would no longer be flying bombs. And attacks like the one on Sept.11,2001 would not be nearly as devastating.

I like this idea. A lot.