12 O'Clock High

From: Wikipedia.org
The series follows the missions of the 918th Bomb Group stationed at Archbury, England during World War II. For the first season many of the characters from the movie were retained, including Brigadier General Frank Savage, Major Harvey Stovall, Major Cobb, Doc Kaiser and General Pritchard, albeit played by different actors than in the movie. In addition to these characters several other infrequently reappearing characters were introduced, including Captain (later Major) Joe Gallagher who appeared twice.

At the end of the first season studio executives decided a younger looking lead actor was needed. In the first episode of the second season General Savage was killed in action and replaced by Joe Gallagher, now a full Colonel. Ironically, the younger looking Burke was two years older than Lansing. According to Executive Producer Quinn Martin(obtained from his Wikipedia entry) he had decided to fire Lansing because he had become difficult to work with.

For the second season most of the supporting cast from the first season was replaced with the exception of Major Stovall, Doc Kaiser and an occasional appearance by General Pritchard. Other actors who did reappear after the first season played other characters. This trend continued throughout the other seasons. Edward Mulhare appeared twice, as two different German officers. Bruce Dern appeared four times in three different roles. Tom Skerritt appeared five times, each time in a different role.

The first two seasons were filmed in black and white. This was done mostly to allow the inclusion of actual WWII combat footage, negating the need for special effects. The inclusion of combat footage in the series was often obvious as it was often quite degraded. Limited usable combat footage often resulted in the same shot being reused in multiple episodes. For the third season the series was filmed in color but only ran 17 episodes, being cancelled in mid-season. Some of the combat footage used for the third season seemed to be black and white footage tinted blue.