The reconfigured F-35 Block 4: major delays, increased costs, reduced capabilities, first deliveries expected only from 2031 onwards.
Background to the F-35 program
The F-35 Lightning II remains the cornerstone of US and allied combat aviation. More than 1,000 units have already been delivered, but the program has been plagued by delays, cost overruns, and high industrial complexity. The Block 4 modernization was initially intended to offer 66 new features, ranging from the integration of additional weapons to improved sensors. This figure then rose to nearly 80, further delaying the schedule.
According to the Government Accountability Office (GAO), full service entry will not occur until the mid-2030s. The Pentagon has therefore decided to refocus the scope on the features deemed most urgent, with delivery scheduled for 2031 at the earliest.
The initial ambitions of Block 4
Block 4 was intended to transform the F-35 by enhancing its technological superiority. The objectives included:
- advanced electronic warfare,
- more efficient sensor fusion,
- the integration of modern weapons such as the British SPEAR 3 missile,
- improved communications and navigation,
- optimization of targeting systems.
These developments were based on the prior implementation of Technology Refresh 3 (TR-3), a set of hardware and software serving as a foundation. Without TR-3, the functions planned for Block 4 cannot be deployed.
Accumulated delays
The modernization was supposed to be operational in 2026. This deadline was first pushed back to 2029, then further. The most recent assessments indicate that it will be mid-2030s before all the features are available.
Several factors are to blame:
- TR-3: delayed by three years, it will not be fully delivered until 2026, instead of 2023. Its development has suffered from difficulties related to the central processor and software stabilization.
- Slowed production: in 2024, Lockheed Martin delivered 110 aircraft, all of which were behind schedule, with an average delay of 238 days. In comparison, in 2021 only 22 out of 142 aircraft were delayed, by an average of 16 days.
- Supply chain: in February 2025, more than 4,000 missing parts were identified on the final line, twice the historical average. TR-3 components accounted for a large part of these shortages.
Considerable cost overruns
Block 4 was initially estimated to cost $10.6 billion. In 2021, this budget rose to $16.5 billion, an increase of 56%. The revised estimate expected at the end of 2025 could further increase the bill.
More broadly, the costs of the F-35 program now total $485 billion for acquisition, up from $442 billion a year earlier. Taking into account maintenance over several decades, the bill exceeds $2 trillion.
The F-35’s engine, produced by Pratt & Whitney, is a major factor in this increase. Accelerated wear and tear due to the power and cooling requirements of Block 4 adds approximately $38 billion to the life cycle cost.
The effects of TR-3
TR-3 is an essential requirement for Block 4 modernization. It includes a new central processor, improved cockpit displays, and updated software architecture.
In 2023, delays led to a one-year suspension of aircraft deliveries. Dozens of completed aircraft were stored without being accepted by the Pentagon. To avoid excessive backlog, a provisional version of the software was finally approved, allowing the aircraft to be accepted but without combat capability.
Today, Lockheed Martin plans to integrate TR-3 starting in 2026, which will determine the rest of the schedule.
Reducing the scope
Faced with the scale of the delays, the Pentagon has chosen to prioritize. Functions requiring an improved engine or deemed less strategic have been removed or postponed. The refocused Block 4 will retain:
- enhanced electronic warfare capabilities,
- improved navigation,
- improvements in secure communications,
- the integration of existing and validated weapons.
This approach will allow for partial commissioning as early as 2031, at least six years behind the initial schedule.
Industrial and financial consequences
The GAO pointed to an inappropriate bonus system: Lockheed could receive performance bonuses even when delivering aircraft up to 60 days late. In 2024, despite an average delay of 238 days, certain financial incentives were still paid.
In addition, Pratt & Whitney did not deliver its engines on time in 2023 and 2024, with an average delay of 155 days. Although this did not halt production, it added pressure to the supply chain.
Implications for allies
Partner countries, including the United Kingdom, Canada, Norway, Italy, and Poland, are waiting for these developments to align their own fleets. The delay is therefore affecting the operational planning of several air forces.
A country such as Finland, which recently chose the F-35, will have to make do with a version whose promised capabilities will only be fully available a decade after its order.
A pragmatic outlook
Block 4 will not achieve its initial ambitions. Nevertheless, the revised strategy makes the program more realistic. Priorities have been defined, the schedule tightened, and objectives aligned with industrial capabilities.
The question remains one of cost: with expenditures already exceeding $2 trillion over the entire life cycle, the margin for political and budgetary tolerance is shrinking. For allies, gradual modernization means having to adapt their doctrine and wait longer before exploiting the aircraft’s full potential.
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