Exhibit 10.21 CERTAIN IDENTIFIED INFORMATION HAS BEEN EXCLUDED FROM THE EXHIBIT BECAUSE IT IS BOTH NOT MATERIAL AND IS THE TYPE THAT THE REGISTRANT TREATS AS PRIVATE OR CONFIDENTIAL. OMITTED INFORMATION HAS BEEN REPLACED WITH ASTERISKS. Supplemental Agreement No. 3 to Purchase Agreement No. 3860 between The Boeing Company and United Airlines, Inc. Relating to Boeing Model 787 Aircraft THIS SUPPLEMENTAL AGREEMENT, entered into as of July 22, 2014, 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. 3860 dated September 27, 2012, as amended and supplemented (Purchase Agreement), relating to the purchase and sale of Boeing Model 787 aircraft (Aircraft). This Supplemental Agreement is an amendment to the Purchase Agreement; WHEREAS, Boeing and Customer agree to substitute *** Model 787-*** aircraft (787-*** Aircraft) in lieu of Model 787-*** aircraft (787- *** Aircraft) and to revise the scheduled delivery as follows: Manufacturer Serial Number 787-*** Aircraft Delivery Month 787-*** Aircraft Delivery Month *** *** *** UAL-PA-3860 SA-3, Page 1 BOEING / UNITED AIRLINES, INC. PROPRIETARY WHEREAS, Boeing and Customer agree to revise the scheduled delivery of *** model 787-9 Aircraft as follows: Manufacturer Serial Number Original Aircraft Delivery Month New Aircraft Delivery Month *** *** *** NOW THEREFORE, in consideration of the mutual covenants herein contained, the parties agree to amend the Purchase Agreement as follows: 1. Table of Contents and Articles. The Table of Contents, is replaced in its entirety with the Table of Contents that is attached hereto and references this Supplemental Agreement No. 3. 2. Tables. 2.1. *** 2.2. Table 1 for Model 787-9 Aircraft is replaced in its entirety with the Table 1 for Model 787-9 that is attached hereto and references this Supplemental Agreement No. 3. 2.3. Table 1 for Model 787-10 Aircraft is replaced in its entirety with the Table 1 for Model 787-10 that is attached hereto references this Supplemental Agreement No. 3. 3. Letter Agreements. 4. Attachment B-2 to Letter Agreement UAL-PA-03860-LA-1209265R1 is deleted in its entirety. 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. [The rest of the page is intentionally blank. Signature page follows.] UAL-PA-3860 SA-3, Page 2 BOEING / UNITED AIRLINES, INC. PROPRIETARY EXECUTED IN DUPLICATE as of the day and year first written above. THE BOEING COMPANY UNITED AIRLINES, INC. /s/ Patrick McKelvey /s/ Gerald Laderman Signature Signature Attorney-in-Fact Senior Vice President – Finance, Procurement and Treasurer Title Title UAL-PA-3860 SA-3, Page 3 BOEING / UNITED AIRLINES, INC. PROPRIETARY TABLE OF CONTENTS ARTICLES SA NUMBER Article 1. Quantity, Model and Description SA-1 Article 2. Delivery Schedule SA-1 Article 3. Price SA-1 Article 4. Payment SA-1 Article 5. Additional Terms SA-1 TABLE 1. 787-8 with GENX-1B*** Engines Aircraft Information Table *** SA-3 1. 787-9 with GENX-1B*** Engines Aircraft Information Table SA-3 1. 787-10 with GENX-1B*** Engines Aircraft Information Table SA-3 EXHIBITS A1. 787-8 Aircraft Configuration A2. 787-9 Aircraft Configuration A3. 787-10 Aircraft Configuration SA-1 B. Aircraft Delivery Requirements and Responsibilities SUPPLEMENTAL EXHIBITS AE1. Escalation Adjustment/Airframe and Optional Features BFE1. BFE Variables SA-1 CS1. Customer Support Document EE1. Engine Escalation/Engine Warranty *** SLP1. Service Life Policy Components TABLE OF CONTENTS, CONTINUED LETTER AGREEMENTS SA NUMBER UAL-PA-03860-LA-1209247 787 e-Enabling UAL-PA-03860-LA-1209264 Open Configuration Matters P.A. 3860 TABLE OF CONTENTS, Page 1 of 3 SA-3 BOEING/UNITED AIRLINES, INC. PROPRIETARY

UAL-PA-03860-LA-1209265R1 Option Aircraft SA-1 Attachment A, 787-*** Option Aircraft Delivery, Description, Price and Advance Payments (DELETED) SA-1 Attachment B-1, 787-*** with GENX-1B*** Engines: Option Aircraft Delivery, Description, Price and Advance Payments SA-1 Attachment B-2, 787-*** with TRENT1000-*** Engines: Option Aircraft Delivery, Description, Price and Advance Payments (DELETED) SA-3 UAL-PA-03860-LA-1209409 Spare Parts Initial Provisioning UAL-PA-03860-LA-1209410 Special Matters Relating to COTS Software and End User License Agreements UAL-PA-03860-LA-1209411 Special Terms – Seats and In-flight Entertainment UAL-PA-03860-LA-1209417 Model 787 Post-Delivery Software & Data Loading CONFIDENTIAL LETTER AGREEMENTS UAL-PA-03860-LA-1209236R1 Model Substitution SA-1 Attachment A, 787-9 Airframe Pricing of Substitution Aircraft with General Electric GEnx-1B*** and Rolls Royce Trent 1000-*** engines SA-1 Attachment B, 787-10 with General Electric GEnx-1B*** and Rolls Royce TRENT 1000-*** Engines SA-1 Attachment C, 787-8 with General Electric GEnx-1B*** and Rolls Royce TRENT 1000-*** SA-1 P.A. 3860 TABLE OF CONTENTS, Page 2 of 3 SA-3 BOEING/UNITED AIRLINES, INC. PROPRIETARY TABLE OF CONTENTS, CONTINUED CONFIDENTIAL LETTER AGREEMENTS, continued SA NUMBER UAL-PA-03860-LA-1209412 Spare Parts Commitment UAL-PA-03860-LA-1209413R1 Special Matters SA-1 UAL-PA-03860-LA-1209414 Other Special Matters UAL-PA-03860-LA-1209413A1 Other Special Matters—Amendment 1 SA-1 UAL-PA-03860-LA-1209416R1 Promotional Support SA-2 UAL-PA-03860-LA-1209430 Performance Guarantees UAL-PA-03860-LA-1209455 *** TERMINATED SA-1 UAL-PA-03860-LA-1209429 *** UAL-PA-03860-LA-1209618R1 Alternate Engine Selection SA-1 6-1162-ELP-0794 *** Program*** 6-1162-ELP-0795 *** Program*** UAL-PA-03860-LA-1301368 Performance Guarantees (787-10) SA-1 UAL-PA-03860-LA-1301373 787-10 Aircraft Open Configuration and Other Matters SA-1 UAL-PA-03860-LA-1301375 Provisions Relating to Customer’s *** for 787-10 Aircraft SA-1 UAL-PA-03860-LA-1301377 787-10 *** SA-1 UAL-PA-03860-LA-1301380 787-10 Program Launch SA-1 P.A. 3860 TABLE OF CONTENTS, Page 3 of 3 SA-3 BOEING/UNITED AIRLINES, INC. PROPRIETARY SUPPLEMENTAL AGREEMENTS DATED AS OF Supplemental Agreement No. 1 June 17, 2013 Supplemental Agreement No. 2 December 16, 2013 Supplemental Agreement No. 3 July 22, 2014 P.A. 3860 TABLE OF CONTENTS, Page 1 of 3 SA-3 BOEING/UNITED AIRLINES, INC. PROPRIETARY Table 1 to Purchase Agreement No. 3860 *** 787-9 *** Aircraft Executed via SA-1 FOR APBP PURPOSES ONLY: UNTIL *** PRIOR TO APPLICABLE AIRCRAFT DELIVERY Airframe Model/MTOW: 787-8 *** pounds Detail Specification: *** Engine Model/Thrust: GENX-1B*** *** pounds Airframe Price Base Year/Escalation Formula: *** *** Airframe Price: $ *** Engine Price Base Year/Escalation Formula: *** *** Optional Features: $ *** Sub-Total of Airframe and Features: $ *** Airframe Escalation Data: Engine Price (Per Aircraft): $ *** Base Year Index (ECI): *** Aircraft Basic Price (Excluding BFE/SPE): $ *** Base Year Index (CPI): *** Buyer Furnished Equipment (BFE) Estimate: $ *** Engine Escalation Data: Seller Purchased Equipment (SPE) Estimate: $ *** Base Year Index (ECI): *** In Flight Entertainment (IFE) Fixed$: $ *** Base Year Index (CPI): *** Delivery *** Prior to Applicable Number of Escalation Factor Escalation Factor Serial Escalation Estimate Adv Payment Base Advance Payment Per Aircraft (Amts. Due/Mos. Prior to Delivery): Date Delivery Aircraft (Airframe) (Engine) Number Price Per A/P *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** Total *** * *** Escalation Factors *** ** *** Escalation Factors *** Boeing Proprietary Table 1 787-9 - APBP > *** Prior Page 1, SA-3

Table 1 to Purchase Agreement No. 3860 787-9 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments APBP beginning *** prior to applicable delivery for *** 787-9 aircraft only Airframe Model/MTOW: 787-9 ***pounds Detail Specification: *** Engine Model/Thrust: GENX-1B***¹ *** pounds Airframe Price Base Year/Escalation Formula: *** *** Airframe Price: $ *** Engine Price Base Year/Escalation Formula: *** *** Optional Features: $ *** Sub-Total of Airframe and Features: $ *** Airframe Escalation Data: Engine Price (Per Aircraft): $ *** Base Year Index (ECI): *** Aircraft Basic Price (Excluding BFE/SPE): $ *** Base Year Index (CPI): *** Buyer Furnished Equipment (BFE) Estimate: $ *** Engine Escalation Data: In-Flight Entertainment (IFE) Estimate: $ *** Base Year Index (ECI): *** Refundable Deposit/Aircraft at Proposal Accept: $ *** Base Year Index (CPI): *** Delivery *** Prior to Applicable Number of Escalation Factor Escalation Factor Serial Escalation Estimate Adv Payment Base Advance Payment Per Aircraft (Amts. Due/Mos. Prior to Delivery): Date Delivery Aircraft (Airframe) (Engine) Number Price Per A/P *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** Total *** ¹ *** on a *** to delivery GEnx-1B*** thrust rating at the GEnx-1B*** price. * *** Escalation Factors *** ** *** Escalation Factors *** Boeing Proprietary Table 1 787-9 with GENX-1B*** Engines Page 1, SA-3 Table 1 to Purchase Agreement No. 3860 787-10 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments (787-10/GE/***) Airframe Model/MTOW: 787-10 *** pounds Detail Specification: *** Engine Model/Thrust: GENX-1B***¹ *** pounds Airframe Price Base Year/Escalation Formula: *** *** Airframe Price: $ *** Engine Price Base Year/Escalation Formula: *** *** Optional Features: $ *** Sub-Total of Airframe and Features: $ *** Airframe Escalation Data: Engine Price (Per Aircraft): $ *** Base Year Index (ECI): *** Aircraft Basic Price (Excluding BFE/SPE): $ *** Base Year Index (CPI): *** Buyer Furnished Equipment (BFE) Estimate: $ *** Engine Escalation Data: Seller Purchased Equipment (SPE) Estimate: $ *** Base Year Index (ECI): *** In Flight Entertainment (IFE) Fixed$: $ *** Base Year Index (CPI): *** Delivery Number of Escalation Factor Escalation Factor Serial Escalation Estimate Adv Payment Base Advance Payment Per Aircraft (Amts. Due/Mos. Prior to Delivery): Date Aircraft (Airframe) (Engine) Number Price Per A/P *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** Boeing Proprietary 787-10 with GE Engines Table 1 to SA-3, Page 1 Table 1 to Purchase Agreement No. 3860 787-10 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments (787-10/GE/***) Delivery Number of Escalation Factor Escalation Factor Serial Escalation Estimate Adv Payment Base Advance Payment Per Aircraft (Amts. Due/Mos. Prior to Delivery): Date Aircraft (Airframe) (Engine) Number Price Per A/P *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** *** *** *** *** ** *** *** *** *** *** *** Total *** ¹*** on a *** to delivery GEnx-1B*** thrust rating at the GEnx-1B*** price. * *** Escalation Factors *** ** *** Escalation Factors *** Boeing Proprietary 787-10 with GE Engines Table 1 to SA-3, Page 2