The way Filipino people dress has been very much influenced by the Spanish Culture. Although in 1940, the Americans have been here for about 40 years and pretty much have also transferred some of its culture to the Philippines. At that time, the Philippine Islands is considered as very culturally advanced. Here are some ideas on how Filipinos dress in those times:
Men:
Everyday wear:
White long-sleeved shirts with so-called chinese collar
Dark loose long pants
Hat made from leaves of local palm
Barefoot or with slippers/sandals
http://www.filipinoheritage.com/costumes/camisadechino.htm
Formal wear:
Barong Tagalog
Dark loose long pants
Or Suit and Tie (same as being used in the US at that time)
Shoes
http://www.filipinoheritage.com/costumes/barong_tagalog.htm
Women:
Everyday wear:
Simple spanish-style (Maria Clara) overall dress or Simple blouse with wrap-around skirt
http://www.filipinoheritage.com/costumes/dress4.htm
http://www.filipinoheritage.com/costumes/dress3.htm
http://www.filipinoheritage.com/costumes/tapis.htm
Formal wear:
Maria Clara or Formal Gown
http://www.filipinoheritage.com/costumes/trajede_mestiza.htm
http://www.filipinoheritage.com/costumes/dress4.htm
Good luck.