[303rd-Talk] Question about crews and "assigned" planes

[Bill Twomey]

I was navigating  around  the  303rd  web site  looking  at  missions  my  father  flew.
Specifically,  looked  at  mission  244.
I  noticed  that  my  Dad,  John Twomey,  wasn't  in  the  plane  often  referred  to  as
that  of  the  "Twomey Crew"   the  "Neva-The Silver Lady."    
This  particular  mission  that  plane  was  flown  by  Jack Rencher  and  Goering.
 
Why  did  crew's  change  planes?   Did  they  do  this  routinely,  or  were  most  crews
assigned  to  a  given  bird?
 
Thanks,
Bill