Exhibit 10.4

CONFIDENTIAL MATERIAL APPEARING IN THIS DOCUMENT HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION IN ACCORDANCE WITH THE SECURITIES ACT OF 1933, AS AMENDED, AND RULE 24B-2 PROMULGATED THEREUNDER. OMITTED INFORMATION HAS BEEN REPLACED WITH ASTERISKS.

Supplemental Agreement No. 3

to

Purchase Agreement No. 03776

between

The Boeing Company

and

United Airlines, Inc.

Relating to Boeing Model 737-9 Aircraft

THIS SUPPLEMENTAL AGREEMENT, entered into as of May 26, 2015, by and between THE BOEING COMPANY (Boeing) and UNITED AIRLINES, INC. (a Delaware corporation formerly known as Continental Airlines, Inc. and successor by merger to United Air Lines, Inc.) (Customer);

WHEREAS, the parties hereto entered into Purchase Agreement No. 03776 dated July 12, 2012, as amended and supplemented (Purchase Agreement), relating to the purchase and sale of Boeing model 737-9 aircraft (Aircraft). This Supplemental Agreement is an amendment to the Purchase Agreement;

WHEREAS, Boeing and Customer agree to *** the following *** firm model 737-9 Aircraft and *** them with *** new model 737-9 *** Aircraft in the same delivery months:

WHEREAS, Boeing and Customer agree to revise the identified manufacturer serial number for the following nine (9) firm model 737-9 Aircraft:

WHEREAS, Boeing and Customer agree to *** firm model 737-9 Aircraft *** under this Supplemental Agreement No. 03 *** for the *** new model 737-9 *** Aircraft *** under this Supplemental Agreement No. 03 and *** Customer’s *** for 737 aircraft.

Supplemental Agreement No. 3 to

Purchase Agreement No. 03776

NOW THEREFORE, in consideration of the mutual covenants herein contained, the parties agree to amend the Purchase Agreement as follows:

The “Table of Contents” is deleted in its entirety and replaced with the attached “Table of Contents” (identified by “SA-3”).

“Table 1”, 737-9 Aircraft Delivery, Description, Price and Advance Payments, is deleted in its entirety and replaced with the attached “Table 1”, 737-9 Aircraft Delivery, Description, Price and Advance Payments, (identified by “SA-3”).

“Attachment A to UCH-PA-03776-LA-1207644” is deleted in its entirety and replaced with the attached “Attachment A to UCH-PA-03776-LA-1207644” (identified by “SA-3”).

The Purchase Agreement will be deemed to be supplemented to the extent herein provided as of the date hereof and as so supplemented will continue in full force and effect.

EXECUTED IN DUPLICATE as of the day and year first written above.

TABLE OF CONTENTS

Table 1 To

Purchase Agreement No. 03776

737-9 Aircraft Delivery, Description, Price and Advance Payment

Table 1 To

Purchase Agreement No. 03776

737-9 Aircraft Delivery, Description, Price and Advance Payments

Table 1 To

Purchase Agreement No. 03776

737-9 Aircraft Delivery, Description, Price and Advance Payments

Table 1 To

Purchase Agreement No. 03776

737-9 Aircraft Delivery, Description, Price and Advance Payments

Table 1 To

Purchase Agreement No. 03776

737-9 Aircraft Delivery, Description, Price and Advance Payment

Note: The estimated Advance Payment Base Prices have been calculated using a ***, and ***, as follows:

Attachment A to Letter Agreement No. UCH-PA-03776-LA-1207644

737-9 Option Aircraft Delivery, Description, Price and Advance Payments

Attachment A to Letter Agreement No. UCH-PA-03776-LA-1207644

737-9 Option Aircraft Delivery, Description, Price and Advance Payments

Attachment A to Letter Agreement No. UCH-PA-03776-LA-1207644

737-9 Option Aircraft Delivery, Description, Price and Advance Payments

Note: The estimated Advance Payment Base Prices have been calculated using a *** and ***, as follows: