The first test flight was February 9, 1969.
That first plane, 747-001, is on display at the Boeing Museum of Flight here in Seattle. You can go on board and see fascinating exhibits about the testing procedures. Usually there is a docent you can talk to as well. They are having a celebration of the birthday at the museum today, but attendance will be sparse as we have a foot of new snow, and driving is strongly discouraged.
The 747 is still my favorite plane; I think they are elegant-looking and very comfortable to fly. And I really like having 4 engines carrying me across the Pond. Fortunately British Airways still has a few flying, including on the Seattle to London route.