The maniraptorans had the necessary equipment.
To me, there seems to be strong evidence that the maniraptoran relatives of birds already had the necessary equipment to move their arms much like the flight stroke. It would have taken only a larger airfoil and a slight change in direction to give this airfoil lift and thrust. And they could run fast enough to get the groundspeed that would have made takeoff easy.

There are two main ideas about how flight evolved.
Archaeopteryx was found in 1861, and since 1880 there have been two main ideas about how flight evolved: "from the trees down" and "from the ground up." Both ideas make sense, at least theoretically. But what actually happened?

The classic "trees down" idea says that the ancestors of birds first started to climb trees, then jump between trees, then glide using small feathered skin flaps, then flap their wings. That's a good, logical-sounding progression. It might even be right.

The classic "ground up" idea says that the ancestors of birds ran along the ground, jumping into the air to catch insects in their mouths, or to avoid predators. They may have used their arms for balance while they jumped, and if there were small feathers on these arms, they would have had some aerodynamic effect. So, little by little, flapping these arms during jumps, birds got up into the air. That's also a good, logical-sounding progression, and it might even be right.

 

Both main ideas have problems: reality is more complex.
The problem is that neither of the classic models deals with the central problem of evolving flight, which is: how do you evolve a flight stroke? If you don't know that, you can't solve the problem of the evolution of flight, no matter where it took place. So "trees down vs. ground up" is a false dichotomy.

But there's another problem. We're probably never going to get fossils of bird ancestors trapped climbing trees and jumping off, or of them running along the ground. So how do we solve this problem? There is considerable argument about whether Archaeopteryx and other theropods show any adaptations for climbing or living in trees. (This is not really a problem: humans and goats can climb trees without any obvious adaptations for doing so.) On the other hand, everyone agrees that theropod dinosaurs were good runners. Maybe this evidence favors a "ground up" model, but the situation is way more complex.