A quick Google Image search leads me to believe that it's a Paulownia tomentosa, more commonly known as a Princess tree. Because of my vantage point on the bridge, I didn't really have a chance to photograph the leaves or other parts of the tree, so I plan return to confirm positive identity. But assuming it is a Princess tree, how cool is that?
And here's your fun fact about the Princess Tree:
In China, an old custom is to plant an Empress Tree when a baby girl is born. The fast-growing tree matures when she does. When she is eligible for marriage the tree is cut down and carved into wooden articles for her dowry.
This page also mentions that tradition, and includes this tidbit
– With the Jewish culture:
“In ancient Israel, a tree was planted when a child was born—a cedar for a boy, a cypress for a girl. As the children grew up, they cared for their own trees” – Tree planting ritual
Here's more on that tradition.
For the record, flowers are edible.
What a find!
Update: Last night I explicitly ran by the tree above to grab some more photos. Here's a few I snapped:
Comparing these to other sites on the web confirm (at least to me), that this is Princess Tree. Having now found this sample, I've actually seen them all over the area. There's a number of them that I've passed dozens of times along GW Parkway. With the bright purple flowers, they're actually hard to miss, once you know what you're looking for.
So yeah, the DC area has Princess Trees. Plenty of them. Amazing. Absolutely amazing.