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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549
 FORM 10-K (Mark One) þ
    
    ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
SECURITIES EXCHANGE ACT OF 1934
 For the fiscal year ended December 31, 2004 OR o
    
    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
SECURITIES EXCHANGE ACT OF 1934
 For the transition period from           to Commission file number 333-110499-37 Principal Life Income Fundings Trust 2004-31 (by Principal Life
Insurance Company as depositor)
  Iowa
(State or other jurisdiction of
incorporation or organization)
    
    42-0127290
(I.R.S. Employer Identification No.)
 Principal Life Income Fundings Trust 2004-31
c/o Principal Life Insurance Company, as depositor
711 High Street
Des Moines, Iowa 50392-0001
(515) 247-5111
(Address of principal executive offices) Registrants telephone number, including area code: (515) 247-5111 Securities registered pursuant to Section 12(b) of the Act: None Securities registered pursuant to Section 12(g) of the Act: None Indicate by check mark whether the registrant (1) has filed all reports required to be filed by
Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for
such shorter period that the registrant was required to file such reports), and (2) has been
subject to such filing requirements for the past 90 days. Yes þ No o Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is
not contained herein, and will not be contained, to the best of registrants knowledge, in
definitive proxy or information statements incorporated by reference in Part III of this Form 10-K
or any amendment to this Form 10-K. þ Indicate by check mark whether the registrant is an accelerated filer (as defined in Rule 12b-2 of
the Act). Yes o No þ State the aggregate market value of the voting and non-voting common equity held by non-affiliates
computed by reference to the price at which the common equity was last sold, or the average bid and
asked price of such common equity, as of the last business day of the registrants most recently
completed second fiscal quarter. None. (APPLICABLE ONLY TO CORPORATE REGISTRANTS) Indicate the number of shares outstanding of each of the registrants classes of common stock, as
of the latest practicable date. Not applicable. DOCUMENTS INCORPORATED BY REFERENCE None.




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PART I


TABLE OF CONTENTS  PART IItem 3. Legal Proceedings.Item 4. Submission of Matters to a Vote of Security Holders.PART IIItem 5. Market for Registrants Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities.PART IIIItem 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters.Item 13. Certain Relationships and Related Transactions.PART IVItem 15. Exhibits, Financial Statement Schedules, and Reports on Form 8-K.SIGNATURESEXHIBIT INDEXSection 302 CertificationAnnual Statement of ComplianceReport of Independent Registered Public Accounting Firm on Compliance    Table of Contents  
 Item 3. Legal Proceedings. None. 
 Item 4. Submission of Matters to a Vote of Security Holders. None. 
 PART II 
 Item 5. Market for Registrants Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities. There is currently no established public trading market for Registrants 4.55%
Principal® Life CoreNotes® due 2010. As of December 31, 2004, there were
less than 300 registered holders of the Notes.   Principal® is a registered service mark of Principal Financial Services, Inc. and is used under license.CoreNotes® is a registered service mark of Merrill Lynch & Co., Inc. 2




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PART III 
 Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters. Not applicable. 
 Item 13. Certain Relationships and Related Transactions. None. 
 PART IV 
 Item 15. Exhibits, Financial Statement Schedules, and Reports on Form 8-K. Exhibit 31.1
    
    Section 302 Certification.Exhibit 99.1
    
    Principal Life Income Fundings Trust 2004-31 made no payment of
interest or principal on its 4.55% Principal® Life
CoreNotes® due 2010 during the fiscal year ended December
31, 2004. As such, no Indenture Trustee Reports were prepared or are
included in this Annual Report on Form 10-K.Exhibit 99.2
    
    Annual Statement of Compliance by Citibank, N.A.Exhibit 99.3
    
    Report of Independent Registered Public Accounting Firm on Compliance.
 The Trusts assets consist of a Funding Agreement issued by Principal Life Insurance Company
(Principal Life) and a Guarantee issued by Principal Financial Group, Inc. (PFG) which fully
and unconditionally guarantees the payment obligations of Principal Life under the Funding
Agreement. PFG is subject to the informational requirements of the Securities Act of 1934, and in
accordance with those requirements files periodic and current reports and other information
(including financial information) with the Securities and Exchange Commission (SEC). You can
read and copy any reports or other information PFG files at the SEC public reference room at 450
Fifth Street, N.W., Washington, D.C. 20549. You can also request copies of PFGs documents upon
payment of a duplicating fee, by writing the SECs public reference room. You can obtain
information regarding the public reference room by calling the SEC at 1-800-SEC-0330. PFGs
filings are available to the public from commercial document retrieval services or over the
internet at http://www/sec/gov. 3




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SIGNATURES Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto
duly authorized. PRINCIPAL LIFE INSURANCE COMPANY
(Registrant)Date: March 29, 2005
    By:
    /s/ Karen A. PearstonName:
    Karen A. PearstonTitle:
    Second Vice President and Counsel 4




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EXHIBIT INDEX ExhibitNumber
    
    DescriptionExhibit 31.1
    
    Section 302 Certification.Exhibit 99.1
    
    Principal Life Income Fundings Trust 2004-31 made no payment of
interest or principal on its 4.55% Principal® Life
CoreNotes® due 2010 during the fiscal year ended December
31, 2004. As such, no Indenture Trustee Reports were prepared or are
included in this Annual Report on Form 10-K.Exhibit 99.2
    
    Annual Statement of Compliance by Citibank, N.A.Exhibit 99.3
    
    Report of Independent Registered Public Accounting Firm on Compliance.
 5