You can approximate this using ITA Matrix. To do this:
- Enter your starting airport.
- For your destination, enter Kansas City (MCI). Then select the "Nearby" link that appears to the right of the entry box, select "2000 miles", and hit the "Select All" check box at the bottom of the pop-up window.
- De-select "Allow Airport Changes". (Otherwise, there's a chance that you'll get a fare that involves arriving at one US airport and departing from another one on the other side of the country.)
The entirety of the continental USA is within 2000 miles of MCI, so your results will include all possible airfares from your starting airport to the continental US. (This excludes Alaska and Hawaii, but I would be surprised if flights there are cheaper than to the continental US, unless perhaps you're starting from Japan and flying to Hawaii.) I just tried this procedure starting from Madrid; the best fare I got was a round-trip to Newark (EWR), a major airport near New York City.
The 2000-mile circle around MCI also includes most of Canada, all of Mexico, and pieces of a few other Central American countries as well, so your results may therefore contain airfares to places like Toronto or Mexico City as well. But these should be easy enough to screen out by hand.
Note that ITA Matrix doesn't link to places where you can buy the tickets. Once you know the fare you're looking for, you'll need to provide the information to a more conventional booking agency such as Google Flights, Kayak, Travelocity, or a travel agency.