I work in the DC area. Right now there is a flowering purple tree (not wisteria).
Every spring someone asks this question, so it's in our FAQ.
The tree that you are describing is Paulownia tomentosa (or Empress Tree, Royal Paulownia). It is exceedingly adaptable, has large leaves, spreads easily, is pretty in bloom but not recommended for residential properties. It is often seen in bloom this time of year, along roadsides or along train track routes. It's also a lovely addition to the plantings at Logan Square in Philadelphia. If you go to Longwood, you will see it there.
Philadelpha biologist David Hewitt wrote a very interesting blog post on the history of Paulownia.
Hope this helps!