It is well known that Pan American World Airways took more than 90,000 reservations for future passenger flights to the Moon from 1968 until its fall into bankruptcy. How far beyond that did Pan Am go? Did they hire engineers to determine, at least, what would be necessary to build a space plane capable of carrying passengers to the Moon? Did any of the aircraft manufacturers start work on the vehicle?
Interesting, been reading about this and it looks like the first moon flight reservation for Pan Am was made in 1964 actually, and then the boom in reservations came in 1968 as you say. *¹
August 1964 Pan Am accepted the reservation of Gerhart Pistor, journalist from Vienna, Austria, as the No. 1 for future flights to the moon. He paid a deposit of 20 US $. About 93.000 people followed on the Pan Am waiting list, called "First Moon Flights Club". Pan Am expected the flight to depart about the year 2000
There's not much out there on the technology, I believe it didn't even made it past marketing maybe, just a publicity stunt or a placeholder *²
A Pan Am spokesperson acknowledged that a few problems would need to be sorted out before lunar flights could get off the ground—among them, the high cost of fuel, passenger comfort during launch, and lack of accommodations on the moon.
New members got a glossy membership card signed by the vice-president of sales and a letter that helpfully explained what would happen next. “Starting date of service is not yet known,” it warned moon-flight-clubbers, but asked for patience while the “essentials are worked out” – something that would surely be happening soon, right?
Also it looks like they had nailed down the looks of the space plane *³
Trans World Airlines was also a competitor collecting about 6,000 reservations for moon flights, which they later canned in a warehouse. *⁴
Trans World Airlines, caught up in the same space exploration fever, also began taking reservations about then. But after collecting about 6,000, TWA found that public interest had waned and now stores the applications in a warehouse.
Finally, here's an interesting thread on History SE about this: How much did Pan Am tickets to the moon cost?
So, tl;dr - I'm pretty certain this did not get past a publicity stunt or just some very early initial artist concept designs (like the one in the picture above).