I've always found strange the willingness of CT believers to use Northwood as some kind of evidence for 9/11. If Northwood proves anything, it is that a government willing to create a casus belli to support a war would never design something like 9/11 to begin with.
Northwood called for relatively small operations: mock attacks, fake bombing and sabotage, involving a very small number of victims (a plane) when there's victims, all far and away from the public eye. All things that make sense; the goal here is to provoke a reaction within the US population, not to kill and destroy indiscriminately. Countless states (including the US) has used similar tactics at one point or another in the past, so by itself Operation Northwood isn't anything special. And note that even with its relatively small scope, Nortwood was actually never implemented by the JFK administration.
9/11 on the other hand was anything but Northwood. We're talking here about thousands of deads, billions in damage, and the destruction of very important US landmarks, affecting the country economy in a very profound and permanent way. In scope, it is many orders of magnitude bigger than Nortwood. If the goal is a mere casus belli, it's a complete overkill. And a risky one too: there're advantages of blowing a plane far and away from the public eye, while doing all these things right in front of everybody is massively stupid when you want to hide a conspiracy. If the Bush government had conspired to create 9/11 (assuming MIHOP here), it would NEVER have done 9/11. Too complicated, too costly, way too risky, and completely unecessary given the stated goal. And why put the blame on Saudis and not on Iraqis? Why destroy the WTC? Why risk that important millitary assets get killed in the Pentagon? There's far too many ways the plan could have seriously backfire for even think about attempting it, when much easier, much less riskier solutions could be easily designed.