UNITED STATES 
                  SECURITIES AND EXCHANGE COMMISSION 
                        Washington, D.C. 20549 
                                    
                                    
                             SCHEDULE 13G 
                                    
               Under the Securities Exchange Act of 1934 
                                    
                                    
                           (Amendment No. )* 
                                    
                                    
                            bluebird bio, Inc. 
                             (Name of Issuer) 
                                      
                               Common Stock 
                      (Title of Class of Securities) 
                                      
                                09609G100 
                              (CUSIP Number) 
                                    
                            December 31, 2018 
         (Date of Event Which Requires Filing of this Statement) 
                                    
                                    
Check the appropriate box to designate the rule pursuant to which this 
Schedule is filed: 
[X] Rule 13d-1(b) 
[ ] Rule 13d-1(c) 
[ ] Rule 13d-1(d) 
 
 
*The remainder of this cover page shall be filled out for a reporting 
person's initial filing on this form with respect to the subject class 
of securities, and for any subsequent amendment containing information 
which would alter the disclosures provided in a prior cover page. 
 
The information required in the remainder of this cover page shall not 
be deemed to be "filed" for the purpose of Section 18 of the Securities 
Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of 
that section of the Act but shall be subject to all other provisions of 
the Act (however, see the Notes). 
                                    
                                    
                                    















CUSIP: 09609G100                                                Page 1 of 4 
<Page> 

                                    
 1   NAMES OF REPORTING PERSONS 
     I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY) 
     Capital International Investors ** 
      
      
 2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE          
     INSTRUCTIONS)                                                 (a)     
                                                                      
                                                                    (b)     
 3   SEC USE ONLY 
      
      
 4   CITIZENSHIP OR PLACE OF ORGANIZATION 
      
     California 

             5   SOLE VOTING POWER 
                   
                  5,284,178 

       
             6   SHARED VOTING POWER 
 NUMBER OF         
   SHARES         NONE 
BENEFICIALL        
 Y OWNED BY 
             7   SOLE DISPOSITIVE POWER 
    EACH           
 REPORTING        5,484,664 
   PERSON 
   WITH: 
             8   SHARED DISPOSITIVE POWER 
                   
                  NONE 
                   
 9   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 
      
     5,484,664          Beneficial ownership disclaimed pursuant to Rule 
     13d-4   


 10  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES 
     (SEE INSTRUCTIONS) 
      
      
 11  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 
      
     10.0%   

 12  TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) 
      
     IA   

** A division of Capital Research and Management Company (CRMC)  






CUSIP: 09609G100                                                Page 2 of 4 
<Page> 

                  SECURITIES AND EXCHANGE COMMISSION 
                         Washington, DC 20549 
                                    
                             Schedule 13G 
               Under the Securities Exchange Act of 1934 
                                    
Amendment No.  
 
Item 1(a)     Name of Issuer: 
       bluebird bio, Inc. 
 
Item 1(b)     Address of Issuer's Principal Executive Offices: 
       60 Binney Street 
       Cambridge, MA 02142 
        
Item 2(a)     Name of Person(s) Filing: 
       Capital International Investors, division of Capital Research 
       and Management Company 
        
Item 2(b)     Address of Principal Business Office or, if none, 
       Residence: 
       11100 Santa Monica Boulevard 
       16th Floor 
       Los Angeles, CA 90025 
        
Item 2(c)     Citizenship:   N/A 
        
Item 2(d)     Title of Class of Securities: 
       Common Stock 
        
Item 2(e)     CUSIP Number: 
       09609G100 
        
Item 3     If this statement is filed pursuant to sections 240.13d-1(b) 
       or 240.13d-2(b) or (c), check whether the person filing is a:  
       (e)      [X]     An investment adviser in accordance with 
            section 240.13d-1(b)(1)(ii)(E). 
        
Item 4     Ownership 
        
       Provide the following information regarding the aggregate 
       number and percentage of the class of securities of the issuer 
       identified in Item 1. 
        
        
       (a)    Amount beneficially owned: 
       (b)    Percent of class: 
       (c)    Number of shares as to which the person has: 
       (i)    Sole power to vote or to direct the vote: 
       (ii)   Shared power to vote or to direct the vote: 
       (iii)  Sole power to dispose or to direct the disposition of: 
       (iv)   Shared power to dispose or to direct the disposition of: 
        
       See page 2  
        
       Capital International Investors divisions of CRMC and Capital 
       Bank and Trust Company, as well as the following CRMC 
       subsidiaries: Capital Guardian Trust Company, Capital 
       International Limited, Capital International Sarl, Capital 
       International K.K. and Capital International, Inc., 
CUSIP: 09609G100                                                Page 3 of 4 
<Page>        collectively provide investment management services under the 
       name "Capital International Investors." Please see Items 5 
       through 11 of each reporting person's cover sheet in this 
       Schedule 13G filing for such person's deemed beneficial 
       ownership of 5,484,664 shares or 10% of the 54,694,411 shares 
       believed to be outstanding. 
        
Item 5     Ownership of Five Percent or Less of a Class.  If this 
       statement is being filed to report the fact that as of the date 
       hereof the reporting person has ceased to be the beneficial 
       owner of more than five percent of the class of securities, 
       check the following: [ ] 
        
Item 6     Ownership of More than Five Percent on Behalf of Another 
       Person: N/A 
        
Item 7     Identification and Classification of the Subsidiary Which 
       Acquired the Security Being Reported on By the Parent Holding 
       Company or Control Person: N/A 
        
Item 8     Identification and Classification of Members of the Group:  
       N/A 
        
Item 9     Notice of Dissolution of Group:  N/A 
        
Item 10     Certification 
        
       By signing below, I certify that, to the best of my knowledge 
       and belief, the securities referred to above were acquired and 
       are held in the ordinary course of business and were not 
       acquired and are not held for the purpose of or with the effect 
       of changing or influencing the control of the issuer of the 
       securities and were not acquired and are not held in connection 
       with or as a participant in any transaction having that purpose 
       or effect. 
        
     Signature 
        
       After reasonable inquiry and to the best of my knowledge and 
       belief, I certify that the information set forth in this 
       statement is true, complete and correct. 
 
        Date:          January 10, 2019 
                        
        Signature:     /s/ Walter R. Burkley 
        Name/Title:    Walter R. Burkley - Senior Vice President 
                       and Senior Counsel, Fund Business 
                       Management Group 
                       Capital Research and Management Company 
 
        
        
        
                
                
                
        
           
                
                
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