Mobile Passport

Yes, Mobile Passport is an absolute time saver It gets you to the front of the line which, as you mentioned, can be pretty long at Dulles.
Download the app before your trip and enter your passport details as well as those for any travel companions.
Once the plane lands turn on your phone and immediately connect to cell service; open the app; select "New Trip" and follow the prompts. Do not wait until you are in the terminal as cell service and wi-fi can be spotty.
Once you have your QR code you are set.
A caution, depending on when your flight arrives 2 hours for a connecting flight is not sufficient. Most of the international flights arrive within the same 2 hour window which puts a strain on the transfer facility; i.e.: long lines at both immigration and through security.

Already downloaded and our Passport data is on it, but it's an iPad, no cellular, we do not use smart phones, has to be their wi-fi.
Two hours, United put this together it was actually 1'45" initially but the connector to Philly was subsequently pushed back. I ahve found in Philly that we are generally out of the airport within 45 minutes of door opening with the Kiosks,in spite of our landing at busy time.

Prior to my recent trip I had installed the mobile passport app. When I landed at Dulles this past Wednesday I atrempted to use the app and submitted my information only to receive an error message. A second submission received another error message and then locked the app so that no other submission could be made. Used the kiosk, and once I got to the CBP agent, he explained that there were all sorts of issues with the app.

More here: https://community.ricksteves.com/travel-forum/tech-tips/mobile-passport-control-app-review

We used it recently when we landed in Boston from Rome. We were in a long line and I had to ask about Mobile Passport twice but the 2nd time we were moved ahead of a long line.

In Boston you have to look carefully for a Mobil passport sign. It is marked on the same little entrance gate as the Global entry.

Irene yes but my experience (in Boston) was that that only helped me pass part of the line. Then I was in the regular line. My one experience didn't really make me a believer, although it was worth a try (it is free after all).

Larry, there is a whole huge thread on Mobile Passport on the Forum from there to or four months ago. Do a search and you'll find a long discussion.

Also I don't know how much you can count on wifi in that part of the airport, but I don't really know.

Mobile Passport didn't work for me last year when returning to SFO from Frankfurt. Couldn't connect to the Wi-Fi, and, like the poster above, the customs agent said there were always problems with it.

All I can report on is Atlanta.. Good Luck. They don't bother to allow the use of the kiosks if the lines get long since being customer service oriented is apparently a violation of their work conditions

Maybe Shelley had problems at SFO last year, but when we came in from Paris in May 2017 we got through everything in less than 15 minutes. Prior trip from Munich in 2016 took slightly longer, but still WAY faster than the people going through the (long) regular line.
Make sure your phone is charged! And if you don't see signs for Mobile Passport, ASK rather than just joining the line (first time, they had to let us through a barrier to get to the MP area.)
Since you will be in the U.S., you can use your domestic plan minutes if you can't find WiFi. I connected from the plane while waiting for the doors to be opened.