I don’t think it was Standard Operation Procedure for B-17 or B-24 crews. However, extra or spare parachutes were always carried by some crews and so individual crews/pilots may have made the decision. Typically in those cases it may have been the job of the flight engineer to check out an extra parachute or two as is mentioned below. Parachutes were “checked out” for each mission and returned upon completion. When an extra was carried, the extra chute or chutes were often in the radio room on a B-17. I have seen other references that put it/them “near the exit”.
Here are some references to the extra or spare parachute.
https://sites.google.com/site/8thafh...ray-peterson-f
“The flight engineer was more or less responsible for the enlisted men. He was a Tech Sergeant and he always carried extra equipment, such as an extra parachute, oxygen masks, medical kits, and so forth.”
http://franckruffino.chez.com/My-Sit...42-38211_3.htm
“The plane carried only one spare parachute, which meant that with the two damaged chutes of the navigator and bombardier, the crew still needed one more parachute.”
http://www.findagrave.com/cgi-bin/fg...l&GRid=3301453
“*He had spilled his parachute in the waist area and was in the process of buckling on the spare parachute.”
You can find many such references to extra or spare parachutes on the web, so certainly for some crews it was standard procedure.
This next reference comes from the “Lead Crew Manual” for the 2nd Bomb Division. (B-24s)
These are items 8 and 9 on the pilot’s before take-off check list. This would suggest that it was written policy for “some” units in the 8th AF, at least to make the effort.
http://www.458bg.com/lead-crew-manual
“*(8)*Extra oxygen masks.
* (9)*One extra parachute whenever possible.”
As a “standard” over-all written policy it may have occurred for B-29s and B-32s (very heavy bombers).
Here is a link to the B-32 Airplane Commander’s Training Manual section on parachutes:
https://archive.org/stream/AirplaneC...nator_djvu.txt
Parachutes
“1. See that each person aboard has a para-
chute on every flight and that there is one extra
parachute aboard for every four persons. As
a minimum, carry two extra parachutes, one in
the forward and one in the aft compartment. “
2. Insist that all persons aboard wear para-
chute harness at all times from takeoff to land-
ing.