Assume that you are using an unlocked smartphone (Android ?) with micro-Sim slot as i5c/s are using nano-Sims ? Do you need voice calling ?
For simplicity & short-term ease of use, both AT&T and T-Mobile has prepaid plan, Sim is typically $9.99 + shipping & tax. Monthly plans with calling minutes (up to unlimited) and up to 3GB (AT&T) and 5GB (T-Mo) of 4G/LTE or HSPA+ speed for as little as $30 USD for 30 days, free & DIY online activation (5 to 10 minutes)
See this link -
http://www.flyertalk.com/forum/trave...ds-usa-14.html
You can order & have the sim shipped ahead to the hotel, check & contact front desk to make sure that they will sign for & hold the package for your arrival. T-Mo will usually get it delivered in 3 to 5 business days with free ground shipping from UPS's distribution hub/warehouse on East Coast.
I leave it up to NOVA folks in metro DC area to discuss signal/coverage of T-Mo vs. AT&T, etc. Or, drive over to the mall and/or corporate store & pick one up but inside IAD, no retailers selling Sim cards that I'm aware of.