Following up on my previous post, here are some maps. The book didn't have anything for Maryland, of course. I do find it interesting where the Outer Beltway is shown crossing the Potomac in the north. Either of the proposed locations would connect nicely with an extension of what is now the Fairfax County Parkway. I've long thought connecting the Parkway across the river to Maryland, perhaps to the ICC, would be a more effective option than extending Route 28 north across the river (setting aside the issue of ramming such a road through residential areas and then through either parkland or Donald Trump's golf course). The Parkway, while not freeway-grade, offers more connections more useful to people driving from one part of the DC area to another than Route 28 does. Using the more easterly alignment shown on the map below would avoid the worst impact to parkland or Trump National, though it couldn't be angled too far east lest it impact Great Falls.
The description of the Outer Beltway reads as follows:
Like several of the other proposed facilities, a previous consideration had been given to another beltway for the metropolitan Washington area. The need for such a beltway in Virginia near the outer boundaries of Fairfax County was confirmed in this study. The Outer Beltway proposed in Virginia is a four-lane, controlled access facility throughout. Interchanges are provided at intersections with major roads except for Route 29-211. There are sufficient alternate means of access to the Outer Beltway in this vicinity to preclude the necessity of an interchange at Route 29-211. A Potomac River crossing on the east in the vicinity of Masons [sic] Neck is contemplated. The Potomac River crossing and its approach in Virginia on the north is to be the subject of further study by the Virginia Department of Highways which prepared the functional plans for the Outer Beltway.
Overview map of Fairfax County with my highlighting added (the dashed area is where the VDH was to perform further study). The Northern Virginia Expressway could not be built today on this alignment because it would run through Huntley Meadows Park, an environmentally-sensitive area. Back in the late 1960s that land still belonged to the US government.

Map showing the southern end of the Virginia portion of the Outer Beltway:

Finally, just because I was so surprised to see it, unrelated to the Outer Beltway was this map of a portion of the Potomac Freeway that would have done a number on the western end of Old Town Alexandria between where the King Street Metro and Joe Theismann's Restaurant are today:

The Potomac Freeway apparently would have connected to another Potomac crossing located just south of the Wilson Bridge and connecting to an extension of I-295 roughly where the Gaylord Hotel is in Maryland today. THAT would have resulted in massively different traffic patterns on a part of the Beltway so many of us grew up despising!
I made some other scans but don't have time to rotate and upload them all just now. One thing I found interesting was that the Outer Beltway was to have a full cloverleaf interchange with the Dulles Access Road. Remember this was long before the Dulles Toll Road was planned. Back then the Access Road did not allow for westbound exits or eastbound entrances (I suppose that's still true, as any such exits/entrances are on the Toll Road except for those two bus-only ramps).