Film is based upon a true incident. The caper was covered in a book called "Nazi Gold: The Sensational Story of the World's Greatest Robbery - and the Greatest Criminal Cover-Up" by Ian Sayer and Douglas Botting. The heist was perpetrated by a combinations of renegade Nazi and American officers. It was also listed as the "biggest" robbery ever in the Guinness Book of Records, in the 1960's.
Donald Sutherland became seriously ill during filming on location in Yugoslavia. His wife received a telegram telling her to come immediately but warning her that he would probably be dead before she arrived.
The number of gold bars stated in the heist is 14,000. The initial value of the stolen bars was estimated at $1.6 million and then later corrected to be estimated at $16 million in total. The international standard weight for gold bars is 400 troy ounces and in 1944, the price of gold was $35 dollars per (troy) ounce. That would make approximated total value of the score to be closer to $196 million dollars; not $16 million dollars as stated.
The "Tiger" tanks used in the film were actually Russian T-34 tanks which had been specially modified to look like Tiger tanks. This is apparent when you look at the suspension of the tanks (T-34s used a modified Christie suspension, whereas the Tigers' wheels were much more elaborate.)
It was during shooting in Yugoslavia 1969, that Donald Sutherland received word, via co-star Clint Eastwood, that his then-wife Shirley Douglas was arrested for trying to buy hand-grenades for the Black Panthers with a personal cheque from an undercover FBI agent. Sutherland recounts this story often, mentioning that when Eastwood got to the part about the personal cheque, he laughed so hard, he fell to his knees, and Sutherland had to help him up. Eastwood then put his arm around Sutherland and walked him down the hill that overlooked the Yugoslav countryside, assuring his friend with complete support of his predicament. Sutherland and Douglas, who are the parents of Kiefer and twin sister Rachel Sutherland, later divorced in 1970.
Clint Eastwood signed to do the film mainly because his friend and favorite director, Don Siegel, was set to direct it. However, Siegel ran into post-production problems while finishing up Two Mules for Sister Sara (1970) and had to withdraw from the project. Brian G. Hutton was then signed to direct. Eastwood, who had already signed a contract to do the film, couldn't pull out.
The German Tiger tank commander (played by Karl-Otto Alberty) appears to be a parody - both in appearance and manner of speaking - of Marlon Brando's portrayal of German Lt. Christian Diestl in THE YOUNG LIONS.
A record was made of Clint Eastwood singing "Burning Bridges", the theme song from the film. It was released as a 45-rpm disc on Certron Records, catalog #C-10010, produced by Dickey Lee and Allen Reynolds (with the B-side of "When I Loved Her" also sung by Eastwood, and written by Kris Kristofferson).
14,000 gold bars, valued at $16M, equals about $1,143 per bar. At the long-standing price of $35/troy ounce, the bars would have had to weigh only 32.6 oz (almost 36 oz avoirdupois) each--probably a gross understatement, even though the bars are clearly much smaller than the standard "Fort Knox" size. But even assuming the weight is correct, 14,000 bars would weigh almost 16 tons (not counting boxes, men and equipment)--well beyond the capacity of the truck they were using. Then again (still with me, folks?), 14,000 bars, at only 12 to a box, would require over 1,100 boxes--seemingly a lot more than is in the pile. So maybe the German colonel was wrong.
A gold bar of 400 Troy ounces would measure roughly 2 inches x 3 inches x 9 inches and would weigh about 28 pounds. 14,000 bars at 28 pounds is 196 tons requiring a minimum of 78 two-and-a-half ton trucks to transport. The bar seen being handed around like it was a loaf of bread looks a bit larger, roughly 3 inches x 4 inches x 12 inches. A gold bar of this size weighs 75 pounds and 14,000 of these bars would weigh 523 tons requiring 209 trucks.
In the nineties, a group of Swedish war game enthusiasts started to build a 1/72 scale model of the town, where the robbery takes place. As they pursued accuracy they even traveled to Yugoslavia and in fact hired a pilot and plane to get aerial photos of the town. Yugoslavian authorities thought they were foreign spies and arrested them. Reportedly, they were released after a couple of hours.