While Spencer Tracy was flying from California to McCall, Idaho, for the shoot, the private plane he was riding in ran out of gas over the remote Owyhee River canyon lands in eastern Oregon. They were forced to land at Hole-In-The-Ground Ranch, a remote ranch in the canyon. Since the ranch had no aviation fuel, the rancher filtered some tractor gas through his felt hat into the plane. With that they were off to McCall without any further problems.
At the beginning of the Revolutionary War, George Washington refused Rogers request to command troops. Washington suspected that after Rogers' long stay in Britain prior to the outbreak of hostilities that he might be a spy. Washington's fears proved correct as an infuriated Rogers formed a group called the "Queen's Rangers" (later the "King's Rangers") and fought on the side of the Canadians against the Colonies. Canada's "Queen York Rangers" claim to be a direct descendant of Rogers' irregular militia.
The most demanding scene for the actors involved the filming of the "human chain" employed by the Rangers to cross a treacherous river. The actors themselves had to do the shots without the benefit of stunt doubles. The sequence was begun at Payette Lake in Idaho but had to be completed in the studio tank because the lake was far too dangerous. For Spencer Tracy, who once complained that the physical labors required of actors "wouldn't tax an embryo," it was his most difficult shoot to that point, surpassing even the taxing ocean scenes of his Oscar-winning Captains Courageous (1937).
The subtitle "Book One: Rogers' Rangers" shows that MGM and King Vidor intended to complete the story in a second film which was never made due to lengthy production obstacles which plagued this film. This explains why the characters never actually make it to the Northwest Passage.
This movie is a pretty faithful telling of Roger's Rangers' raid on the Abenaki town of St. Francis, except for the end. When the Rangers arrived at Fort No. 4 they had missed their resupply by only two hours. Lt. Stephens heard them coming and retreated, thinking the Rangers were French troops. Rogers and a couple of men built a raft and pursued Stephens downriver. They caught up with him and sent the supplies back to No. 4 within 10 days, as Rogers promised. Lt. Stephens was later court-martialed for cowardice and cashiered out of the Rangers.
A young Greer Garson was originally the first choice of MGM to play the alluring character of Elizabeth Towne. A scheduling conflict resulted in the character being recast and velvet-voiced Ruth Hussey being given the role.
Kenneth Roberts, the author of the best-selling 1936 novel on which the film is based, described Rogers Rangers as wearing "green buckskin" clothing. The initial attempts made at dyeing the buckskin uniforms made them a bright Kelly green. They were subsequently re-dyed to attain the forest green color seen in the finished film. The uniforms were occasionally "touched up" with paint spray guns, while being worn, with the actors wearing protective bags on their heads - as seen in Northward, Ho! (1940), a period making of.
Rogers sends by 40 men; which was said to be 20% of his force. Making his force 200 - 40 = 160 men. He loses one to a broken leg, sends four more back with info on where he's going, and lastly he leaves one behind at the river crossing. That left 154 men to cross the river. When they had crossed and Rogers asked for an accounting he was told 142 were present. That would indicate they lost 12 men in the rapids.
W.S. Van Dyke was assigned to direct the movie before King Vidor, and scouted locations throughout the American West in 1938. About 60,000 feet of background shots had been completed in Idaho before Van Dyke was taken off the picture because of a scheduling conflict and replaced by Vidor; Jack Conway eventually shot some additional scenes as well.