The poster tagline was "You'll believe a man can fly", but before December 15, 1978, it was some feat even to imagine how an earnest movie about a man in a red cape and tights could ever get off the ground.
So much was so different then. It was a Hollywood before entire CGI cities and cinematic universes and online fandoms - but also before the discovered charm of Christopher Reeve.
Today, six of the eight top-grossing movies of the year are superhero stories - and this month brings Spider-Man: Into the Spider-Verse (out this weekend in the US) and Aquaman (December 21), plus the PG-13 sequel recut Once Upon a Deadpool.
And when trying to pinpoint just how Hollywood got here - with most of its billions in box-office receipts swaddled in spandex - the releases of Batman (1989) and X-Men (2000), Spider-Man (2002) and Iron Man (2008) stand as crucial mile markers.
Yet director Richard Donner's Superman: The Movie (the 40th anniversary of its opening was on Saturday) is the true father of superhero cinema - the first such project that captured the masses while also refusing to steer hard toward camp (as had Superman and the Mole-Men in the 50s and the TV series-related Batman: The Movie feature in the 60s).
In the years just before the debut of Superman - which has just received a 4K restoration release to mark the anniversary - 1975's Jaws birthed the summer-season blockbuster, and 1977's Star Wars ushered in a new type of mass-merchandise campaign.
So by the winter of 1978 - the year Star Wars and Close Encounters of the Third Kind received effects Oscars - audiences were primed for an epic science-fiction film adapted from a comic book.
Superman, created four decades earlier by Midwestern teenagers Jerry Siegel and Joe Shuster, was a natural candidate - an icon of Americana who also happened to be a superpowered space alien.
Warner Bros, though, wasn't going to blaze any trails without adhering to some of the hidebound rules of making a big studio picture. That meant securing big stars, a hot director and respected screenwriters.
After the huge success of the first two Godfather films, the Superman team went after star Marlon Brando and writer Mario Puzo, and Donner was hired fresh off The Omen (1976).
That's when things began to get bloated. Brando would command a US$3.7 million ($5.4m) payday and a high percentage of the back end for playing Jor-El, Superman's father. And Puzo's screenplay ran hundreds of pages. The production budget would climb to US$55m - one of the most expensive movies ever at the time.
Another A-lister, Gene Hackman, was ultimately landed to play villain Lex Luthor. But who could credibly fill the red, blue and yellow tights?
The name of seemingly every big leading man was mentioned. But the great casting director Lynn Stalmaster urged consideration of Reeve, who rippled with easy charisma.
Reeve also had a ready chemistry with Margot Kidder, who played Lois Lane. (Kidder died in May.)
As mild-mannered reporter Clark Kent, Reeve could do light screwball comedy behind oversized specs - especially after studying Cary Grant in Bringing Up Baby.
Still, the filmmakers worried: Wasn't the 77kg Reeve too thin for the part? Refusing a padded costume, the actor packed on the required kilos with the help of David Prowse, the bodybuilder beneath the Darth Vader suit.
Reeve was hired for US$250,000 - or just about what Brando ultimately made per minute in his limited screen time.
Five writers would lay hands to the screenplay, including Oscar-winning Robert Benton and script doctor Tom Mankiewicz. John Williams would provide the triumphant Oscar-nominated score. And in a pre-CGI era, the practical-effects and blue-screen wizardry would boost Superman to a special achievement Academy Award.
Superman opened in more than 700 US theatres - to mostly positive reviews.
Forty years on, Superman, for all its rough edges, has only burnished its stature as a forerunner - a canary in Hollywood's gold mine of superheroes.