The secret history of McDonald's Filet-O-Fish, which was almost killed from the menu before becoming one of the chain's staple sandwiches (MCD)

• McDonald's Filet-O-Fish sandwich — the first new non-hamburger item added to the fast food giant's menu — went nationwide in 1965.
• It was the brainchild of Cincinnati-based McDonald's franchise owner Lou Groen.
• Groen came up with the idea when he discovered that the Catholic practice of abstaining from meat on Fridays was hurting his business.
• At first, McDonald's CEO Ray Kroc hated the idea of "stinking up" the restaurant with fish.
• He reconsidered when the Filet-O-Fish trounced Kroc's "Hula Burger" sandwich in a head-to-head contest.

Believe it or not, the Filet-O-Fish almost missed the menu.

Nowadays, the sandwich is iconic, and responsible for a whole bunch of piscine imitators. Business Insider's Mary Hanbury reported that the Filet-O-Fish is a massive hit during Lent, when many Catholics fast and abstain from eating meat on Fridays.

It's one of President Donald Trump's favorites, too. He's known to put away two of the fish sandwiches at a time, along with two Big Macs and a large chocolate shake.

But the sandwich's enduring success contrasts with its floundering start. Former McDonald's CEO Ray Kroc initially thought that he had bigger fish to fry when Cincinnati franchise owner Lou Groen first proposed the idea in 1962. 

Here's a look at the early history of the Filet-O-Fish, which owes its briny existence to Cincinnati-based Roman Catholics and the fact that most people don't find pineapple-and-cheese sandwiches all that appealing:

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After seeing a McDonald's ad in a magazine, Groen opened his first golden-arched restaurant in Cincinnati in 1959. He also purchased the franchise rights for the city and northern Kentucky.

Source: Business Insider, "Historic Restaurants of Cincinnati: The Queen City's Tasty History"

McDonald's was far from the only burger joint on the block in those days, and the market was crowded and competitive.

Source: Business Insider, "Historic Restaurants of Cincinnati: The Queen City's Tasty History"

Before the Second Vatican Council took place in the mid-1960s, Roman Catholics were supposed to abstain from eating meat on Fridays.

Source: The Catholic Telegraph, The List Show TV, The Baltimore Sun